I understand that after the Marian reforms that training and arms were standardized for soldiers but before that each soldier had to supply his own arms. The thing is, since the requirements to be a soldier were high for an average person, how much of an army would composed of lightly armored and ill equipped versus the richer and well equipped principes or triarii? Would the generals try to balance the ratios of poor to rich soldiers or did the poorer soldiers compose the wide majority of an army?
As an extension question, how common was cavalry in pre-Marian Roman armies? Equites came from the higher classes and I can't think of horses being cheap so around how many of these men would actually be in a large army?
I'm going to tackle the cavalry question.
Roman (and Italian) citizen cavalry was standard from 300 to about 100 BCE. The Senate attached Roman/Italian cavalry corps numbering 200-300 men in every Roman army, and as much as 900 men if the contingent was Italian (as opposed to citizen Roman). These forces were often supplemented by non-Roman/Italian forces, and our sources are sometimes frustrating in that they refer to these as auxilia, a term which is also applied to Italian forces. Therefore we have a bit of a problem sometimes distinguishing between Italian cavalry and those from outside of Italy.
Eventually, this practice ceased entirely, and no more Roman/Italian cavalry were used with armies. For instance, by the 50s BCE it is pretty clear that Caesar had no significant force of mounted Roman/Italian troopers with him in Gaul. The Roman army had become entirely dependent on allied contributions (Gallic or German or Spanish or Numidian or whatever). We don't know exactly when this transition happens, or precisely why, but it belongs in the early first century BCE. (Note: there is one exception to this, and it is a whopper: at Pharsalus, Pompey had a "great force" of Roman cavalry with him (Frontinus: magna equitum Romanorum manus). I don't really know what to do with that evidence, honestly, and Plutarch corroborates it. Civil war tends to make "standard practice" go out the window.
I think it is important to differentiate the role of a cavalry trooper on the battlefield and the role of a member of the equestrian class. Yes, historically, both in Italy and Greece and Macedonia and Gaul and many other Indo-European societies, the cavalry trooper was such because he could afford to field a horse, a very expensive animal. This tie between the role of a trooper and his socio-economic status becomes less and less valid in the Late Republic. The Gallic cavalry certainly carried with them no elevated social status compared to the citizen legionnaires. The range of wealth and status associated with a Roman "equestrian" is staggering, from some that were quite poor to others that were filthy rich. The horse-origin of the name was very old and barely applicable in the historical period. There were thousands of equestrians in the legions who were not cavalry troopers.
Republican Roman cavalry is a topic that can take up an entire book study, and in fact it has! Jeremiah McCall has a book on this subject (The Cavalry of the Roman Republic Routledge 2002). He gives a very thorough review of all the evidence, pertaining both to the fighting side of cavalry and the socio-economic side of things.
I think you are overestimating the cost of a Roman soldier's equipment before Marius. Polybios describes for us the standard legionnaire's equipment during the Punic Wars period, and gives every indication that virtually every infantry soldier was equipped in a similar way, with minor variations between the divisions of the triplex acies. In most cases the variations seem more tactical than socio-economic. I don't think we should imagine "poor" soldiers on the battlefield with pitchforks and shabby tunics. Also, the definition of "poor" in this instance is very broad. The true "poor" were not enrolled until the initiative of Marius, who enrolled the capiti censi -- those who were "counted by head" because they mattered so little in the elections. Before that, even the least well-off person in the legion would be able to afford the equipment of a legionnaire (or have it deducted from their pay). In fact, the census classified those who could and could not participate in the army before Marius, and in 146 the minimum was 4,000 asses (Polybios 6.19.2), down from 11,000 in the decade before Hannibal's invasion. The minimum was probably reduced further late in the 2nd century to 1,500 asses, though this is a contentious issue (see Cicero Rep. 2.40). In 146, a person with at least 4,000as was an assiduus and eligible for service in the legion; if not, he was one of the proletarii and only eligible in extreme circumstances. An assiduus was certainly not a rich person, necessarily. A small farm was more than enough to qualify most of the time and get all the basics: sword, shield, bronze helmet, a couple of pila, and torso protection of some sort -- maybe not an elaborate lorica.
I should say that it is entirely unclear how the legions were actually furnished with equipment, even in the Late Republican armies of the triumvirs. The standard line is that before that, the soldiers brought their own equipment, but the standardization witnessed by Polybios even during the Punic Wars era seems to contradict that somewhat. There were velites (light-armed troops), but their role, numbers, and prevalence in Roman armies in any period is unclear at best. The core of the force was always the good ol' infantry with gladius, scutum, pilum, galea. If the soldiers were buying swords and armor at a stall at the local market or from the local smith, we don't hear about it in the sources. This post and the follow-ups has some more discussion in that vein.
Cagniart has an interesting little chapter which covers some of the issues here and gives some sourcing and bibliography. That is in the volume edited by Erdkamp in 2007, A Companion to the Roman Army (Blackwell).