As a fellow systems administrator I would never buy production hardware (the R710) off ebay buying new has the advantage of next day replacement parts from dell and it is a great thing. Also with virtualization you want to be careful you do not want to put all your eggs in one basket you traditionally want to run 2 ESX hosts with primary DC on one and a Secondary DC on the other, by running all on one host you run into the problem that if something happens to the hardware all of your hosts are down you may want to look into the idea of n+1 virtualization. Also are you going to be doing a P2V for the conversion or are you going to be recreating the servers from scratch?