I guess I will be the party-pooper voice of dissent here.
There is no way I would spend $1500 on a TB Clone in 357 Sig.
The odds of it running reliably are probably pretty low, especially one sitting on the "used" rack at a local store. Assuming history is any guide you will pull your hair out and throw piles of money at it on the prayer it may function one day. Its not like there is a HK "spec" for 357 sig where you know this combination of parts should work, so you will be troubleshooting without a known good config. I couldn't imagine the pain of troubleshooting a Bailey built 357 Sig MP5 and having to use expensive 357 Sig ammo to do it while swapping out German parts hoping for the right combination.
If it does run now, Coharie 357/40/10 clones had a poor reputation for the longevity of the bullet guides in the trunnion. If one of those little welded in guides breaks, the gun is basically done short of a full trunnion/barrel replacement.
Lets say best case it runs like a raped ape from the get go ...a 357 Sig MP5 for me is an answer to a question nobody asked.
Rebarreling it to a 10 or 40 is also probably a crap shoot. Who knows what spec that trunnion barrel hole is, how wonky the barrel pin hole was drilled, and if you can find a replacement barrel that will install and gap correctly with a new 10 or 40 LP.
If you want a 10 or 40, I would just buy one of those out of the gate vs. hoping a Coharie 357 could be successfully rebarreled if you don't like it.
Just my 2 cents anyway