If we buy a returned EVO from here, which may have ballast problems, among others, will we be able to buy the parts to replace them with? From this site, I mean?

You can buy eVo ballasts and bulbs from LL:

http://lumenlab.com/estore/home.php?cat=254

If you need any other parts your best bet would be to contact LL and ask if they have any. That's sorta why they offered to sell you 2 eVos at a discount.
You might also be able to swap or buy/sell parts you do have with other eVo owners who have also purchased discount eVos.

Quasi has the right idea. Also, the trading post may be a good place to swap "extra" parts that you have with other users.

Ballasts and lamps for the 1.0/1.1 are available directly from the store as well.

I haven't been keeping close tabs on the eVo problems, so I don't know what are the most likely points of failure, however, seeing as how it seemed to be a systemic quality control issue from the Chinese factory, it's more than possible that 2 separate eVos would have the parts needed to frankenstein a working unit from the pair. Hard to say for sure, but that's why they're going at firesale prices, I'd wager. I think it's also more than possible that some of the functional components in a non-working eVo may be usable for other projects as well.

I've picked up a pair of them myself, and have been thinking of picking up a ballast and lamp for potential other uses as well. One such is an attempt to retrofit an eVo light engine into a commercial projector. If I can do that, then the cost of the lamp + ballast is cheaper than the cost of the replacement lamp alone.