There's not really a whole lot to say on the subject, really, in part because of a general shortage of evidence but also because of the nature of the senatorial class. I suspect you're imagining some kind of state-provided public security detail like that protecting officials of modern nation-states or at least the way most people (largely erroneously) imagine the praetorians to have been a bodyguard. Senators were not public officials, but private citizens, and except when members of the senatorial class were holding the elected magistracies of the Republic they were provided no state-supplied subordinates. Even when they were provided with such subordinates they relied extensively on their own private staff, which was larger, often more experienced, and familiar--often in the original sense of the word, being, as mainly slaves, freedmen, and clients, literally members of the household. Senators and the senatorial class were simultaneously private citizens and public personas. They existed as legally private entities but their roles as political advisors to sitting magistrates and the great patrons of the city and Italy made them the most public figures in the Roman world. Moreover, they were among the Roman world's richest individuals, holding land throughout Italy and often (frequently by proxy) in the provinces. They had huge households, armies (sometimes literally) of clients, and could call in favors from across the Roman world. As such, members of the senatorial class were never alone. They were constantly surrounded by a crowd of slaves, freedmen, clients, family, dependents, friends, and other hangers-on, apart from whatever public apparitores they might hold as magistrates. Bigger and better public entourages were signs of greater magnificence in the public sphere, and these retinues could be quite large. Gellius, for example, quotes Asellio as mentioning that Ti. Gracchus was accompanied at all times by a crowd of three or four thousand clients.

Generally speaking as far as "security" went, these entourages, whether part of a senator's household or not, formed the basis of the senatorial class' personal protection. Bodyguards even for magistrates were the stuff of tyranny, and unthinkable in a state-funded form for private citizens. Caesar, for example, dismissed the military guard he was allowed as dictator and conspicuously surrounded himself only with his own friends, staff, and clients. These retinues could and often were turned to violence, either offensively or in defense of their patron. When Pompey accompanied Quintus Cicero to the forum, in the face of threats of physical violence from Publius Clodius, he did so with a veritable private army of veterans, slaves, freedmen, and other dependents, and he was met with Clodius' opposing force, led by a troop of gladiators borrowed from his brother. The mention of gladiators is important. Gladiatorial troops, by the 60s and 50s, are ubiquitous in private retinues, and though there is little direct evidence for their existence in the 70s and 80s I find it hard to believe that they were not widespread that early--see the ludi in Campania, which spawned the Spartacus revolt. Crowds of clients about one's person were not exactly the most secure form of protection, as Livius Drusus was murdered by a man hiding out in his crowd of clients, either in his own home or in the forum. Moreover, they were not reliable--Ti. Gracchus was unable to call his Italian supporters to his aid during his bid for reelection to the tribunate, as they were occupied with the harvest, and was forced to rely on whatever he could scrape together from his own household. And of course they were not especially great fighters in the event that actual mass violence broke out, as both Gracchi learned to their dismay. Gladiators, on the other hand, were professionals, and what's more they were in abundant supply. The social obligation among the wealthy to provide funerary games meant that a constant stream of gladiators was passing through the hands of the political and economic elite, and it was often convenient simply to keep these troops on the payroll, so to speak, more or less indefinitely. Gladiators appear everywhere in private retinues in the 50s and 60s. Both Clodius and Milo turned them against each other, as early as Metellus Nepos they were used, and even politically neutral equites like Atticus had their own private troops of gladiators. Nor were trained professionals of this sort the only source of muscle, but also ordinary thugs, which Sallust mentions could be hired in the streets in pre-organized gangs, either for private protection or offensive action against political opponents.

There's not much that can be said about villas specifically. We simply lack a great deal of evidence--authors don't talk about the staff of villas much. Some things can certainly be said however, both from the texts and from reasonable conjecture. Most members of the senatorial class, and the equites below them, had property holdings throughout Italy or (as Atticus) in the provinces and free cities. Cicero owned eight villas, two houses, several smaller properties, and a great deal of developed land in Italy that he did not use personally but rented to clients and on which he could be certain of lodging should he be travelling in the area. Nor was Cicero considered especially wealthy or lavish in his landholdings, although he was quite proud of his Palatine house. Like many in the senatorial class Cicero spent most of his time touring his estates, during which was of course included a general check on how dependent communities throughout Italy were doing. Villas and other such properties had their own permanent staff, composed of both slaves and free persons (many freedmen), whose duties included the general maintenance of the estate as well as of course any farming ventures undertaken in the adjoining fields. These staffs were well organized, in good Roman fashion, containing field workers, household workers, and other such divisions--we should expect that watchmen and similar individuals made a regular part of the staff. In addition to these, no senator traveled to his estates without a major personal retinue, which would bolster the staff and provide security on the journey. When Publius Clodius was murdered on the Via Appia it was as part of a clash between his private retinue and Milo's, as both happened to run into each other outside the city, on their ways to and from their estates. Indeed, guests usually brought their own retinues, in the same way that they would show up to dinner parties in the city accompanied by a flock of their own household members and clients--to fail to do so was a sort of tacit acknowledgement of public unimportance. Cicero mentions attending a dinner party at Philippus' villa during Saturnalia which Caesar also attended, and remarks that Caesar had brought so many of his soldiers--two thousand, he says!--with him that they had to pitch a camp in the surrounding fields. In the same dinner party Cicero mentions the freedmen and slaves of the household had three full triclinia to themselves in which to eat. Cicero himself mentions that he felt uneasy seeing this gaggle of soldiers fortifying the villa, and Cassius Barba, presumably a staffer or friend of Caesar, provided him with some custodes of his own. This example is, Cicero makes quite clear, obviously exceptional, but this is the sort of way the senatorial class lived.