A bad ESN won't keep the wifi from working. It will just keep Sprint from activating it to their systems. QPST is a great took for "fixing" it, but the newer phones (and I believe the Epic was one) the ESN is a read-only chip. Cant program or overwrite it.
A bad ESN also does not mean the phone was stolen per-se. It could mean that the person skipped out on their contract and didn't pay the ETF yet.
For the WIFI issue, you could try to do a full restore with the ODIN files, that will restore the radio, wifi, bluetooth, but you still won't be able to use it on Sprint.
PM me, I may be interested in throwing you a few $$ to get it off your hands if you can't find a way to use it.