Is it better to Buy a premade or go diy.

If you are buying in large quantities, would it be a better idea to order them from dell or hp, or get bare-bones kits from newegg? We're talking 15-20 normal workplace tower setups, a router, wireless AP, maybe a switch and rack in case of expansion, and maybe a couple of laptops.

As you say "normal workplace towers" I'd say go with a manufactured model. You can probably get a good deal both on the machines and the OS.

I'd tend to lean towards HP at this point but see what they both have to offer.

http://www.hp.com/sbso/

what Jephree said.