The decision of how and where to try Eichmann was a source of significant tension, inside Israel and out.

His capture, which was largely seen in Israel as an act of justice and triumph, galvanized the nation. Nachum Goldmann, for example, was a prominent Zionist who argued that an international tribunal should be used to try Eichmann. Joseph Proskauer, who was formerly President of the American Jewish Committee, worried that the trial would cause anti-Semitism and hostility to Israel in the U.S., and suggested letting West Germany try Eichmann instead (which Israel considered outrageous, given West Germany had a reputation for letting Nazis walk free) or give him over to an international tribunal. But the decision to keep Eichmann for a trial in Israel was as much motivated by politics as by justice; Israel did not trust other states to try him fairly, and also saw this as a twofold political win: on the one hand, Israeli Prime Minister David Ben-Gurion could gain points for his handling of relations with Germany, which had seen him get criticized for being too "conciliatory" in light of the history of the Holocaust, and he could also use the Eichmann trial to really open the nation's eyes to the Holocaust, which was not fully grasped by many Israeli Jews. Ben-Gurion was unabashed about this; he said no punishment or trial could match the enormity of Eichmann's crimes, but said that this trial would "unfold the canvas of atrocities and evil plots of the Nazi regime against our people". This was something not many could stomach disagreeing with, for obvious reasons.

So what about Robert Servatius? Servatius was already famous, given he had a career of defending Nazis, including at the Nuremberg Trials. Servatius was one of the lawyers who volunteered to defend Eichmann, and he was chosen by Eichmann, as you've noted. Israel chose to follow the Nuremberg precedent, though, which meant giving the accused the right to select their own lawyers, rather than an Israeli one in the Eichmann case. It was for this reason that Israel's government was the one paying Eichmann's legal bills as well, rather than Eichmann himself. This was supplemented by the sale of Eichmann's papers and documents, which were sold by François Genoud. The other reason for allowing Eichmann's choice of lawyer was legitimacy; Israel sought to impress upon the world that it had the right to try Eichmann, and Eichmann signed a statement submitting to an Israeli trial so long as he was provided an adequate defense, as reported here in 1960; this preparation to pay for and allow Servatius to act as Eichmann's lawyer was based on this theory of an adequate defense. It was not true that Servatius had no other source of funds, but Israel was not aware at the time of the decision that Eichmann planned to sell documents to fund his defense. An Israeli lawyer was tasked to the team to ensure that they understood matters of procedure in court, also with the goal of ensuring an "adequate defense" and a fair trial, also following the Nuremberg Precedent. Israeli law, by the way, establishes some of the procedures of such a trial as well as the punishment, and while it does lay out the ability to deviate from some rules of evidence for expediency's sake, it does not take away the general presumption that a fair trial must be given, particularly since it was assumed that a Jewish lawyer would be so antagonistic to Eichmann as to make the trial impossible. Even so, Servatius might not have been allowed to represent Eichmann had Israel's inquiries not exonerated him of involvement with Nazis. While it was well-established that Servatius had defended the Nazis at Nuremberg, and while he claimed he defended Eichmann for the money, he was still investigated by Israel for any ties to the Nazi government and for sympathies for the Nazi cause, and only when cleared was he voted on to give him the special legal changes necessary for Eichmann to use him as his lawyer.