Rondo should be worth both Irving and Varejao by himself. No way I'd throw in more. Irving has an injury history and Sideshow Bob is getting up there in age and isn't much of an offensive player. I'd actually try to finagle a conditional pick too from Cleveland if Irving can't play 2/3 the games in his first year. (not sure if that's allowed though)
Irving isn't THAT injury prone. The general consensus is that he is because he got injured in College and missed a lot of the season. He's healthy right now and has way more potential than Rondo. Rondo has had his fair share of injuries, but gambling a top 1 point guard for a top 5 point guard (this season) would make us 6 more seasons competitive theoretically, while giving us no less a chance to win it all this year. Add in Varejao and it should be a no brainer.
Varejao isn't an offensive force, but he is NOT a liability. Just because a player doesn't contribute a lot on offense (which is untrue with Varejao - he gives you hustle points and will convert easy baskets given the chance) doesn't mean he's not a good center. The fact of the matter is that the Celtics need a center that can help them out with interior defense. Varejao is one of the quickest centers in the NBA and will excel with KG, despite being 30. If the Celtics get Varejao, it would be for that reason; not to be a Hakeem caliber low post option or a Karl Malone caliber pick and role player, but someone who'll make our defense elite again.
Rondo isn't worth both Irving and Varejao.