The very first line of Cicero's De Oratore reads as follows
Cogitanti mihi saepenumero et memoria vetera repetenti perbeati fuisse, Quinte Frater, illi videri solent, qui in optima re publica, cum et honoribus et rerum gestarum gloria florerent, eum vitae cursum tenere potueunt, ut vel in negotio sine periculo vel in otio cum dignitate esse possent.
At the beginning, what does "fuisse" refer to? And why isn't "perbeati" declined in the accusative? And is "memoria vetera repetenti" together?