To be honest, with that budget you can probably find a pre-built solution that you can purchase. I really doubt it that someone would build you a custom decentralized exchange from scratch (except maybe the all-knowing Indian scammers who will undoubtedly spam this thread)
I agree, and if the OP don't have a clue on everything what was written in the code, chances are there could be back doors. So just a friendly advise to the OP, just be careful on who you are going to hire to build the DEX that you are dreaming off.