There is a guy in my area selling an evo 3 eprom ecu. I'm prepping to get ecmlink and my buddy told me I could save a bit of money with the eprom because I wouldn't have to get the full ecmlink package.
Does the evo 3 eprom ecu work in a 2g dsm?
If so, what else do I have to buy to get ecmlink with that eprom?

You can only run v3 lite on evo ecu’s, not the full feature version.

You need to re-wire the car for it. Some Evo owners will do the DSM conversion to be able to run ECMLink Full; there isn't much of a point in using the Evo ECU at all.