The 2007 Mac Pro (and perhaps models on either side of it) were notoriously flakey. I was gifted one by a colleague who couldn't bear to fight with it anymore (he had Apple replace various parts and it still restarted randomly, etc.). I learned a lot working on it myself (not as my main machine) and ended up parting it out and shipping pieces all over the country courtesy of ebay...
Anyway, I would be hesitant to put any money into even the later ones (another colleague has a 2009 model that won't boot, even after repairs). I would personally build a Hackintosh out of new parts before buying one of these older Mac Pros. Even if I got another for free I could never trust it.