@Joe ******
I think most agencies charge about 5000₿ for help opening an account.
But if I were you I'd try myself first. Go to some bank branches in the big malls.
Emquartier, Central World, Siam. (If you'll be living in Bangkok)
Tell them you want to transfer 800,000 and they may become receptive.
It's somewhat common for them to say you need to buy an insurance policy from them to open but just say no, and if they insist then leave and try another bank or another branch. Each bank manager makes their own policies (typical in Thailand).
If you can't open one yourself then find outside help.
Then as soon as you get that taken care of, take your proof of overseas transfer 800,000 from the bank to immigration and get your 90 day non-O visa.