The Face-Lifted Latios come with factory installed HU already done in Japan, no longer the JVC/Kenwood HUs that were more flexible in terms of function. The factory built HU can only play CDs and radio. I believe nobody will offer you a trade-in for the factory-built HU unless its a distributor promotion. Anyway, a good choice will be to keep it so you can install it back when you are selling/exporting your car and move your upgraded ICE to your new car.

Just keep in mind a few things when you are choosing a new HU:
Do you have the intention to upgrade the rest of your ICE other than the HU or just want the mp3 function?
If it's the latter, just choose a HU that you like that can play mp3 from some of the more trusted brands eg Alpine, Pioneer etc that meets your budget. There's a wide range of prices for HUs.
If it's the earlier choice, do consider
at least a HU with time-alignment function like the Pioneer P80RSII or the Alpine 9887. Choose a HU/source that have the functions that you want to avoid wasting too much $$ when the upgrade bug bites you.