Banks like proof the funds were in a bank well before closing. 

Just about any bank can open a joint account. 

As a PMC, we allow tenants to:1) Mail in rent (we charge a Processing Fee)2) Deposit at our bank (we charge a Processing Fee, but less than the mailing one)3) Use Zelle, Venmo, etc. 

An offer was received that I accepted. 

I love being on the other side charging everything possible on a credit card but there’s no reason to accept them.

You could get a loan from a local bank or private lender to fund it

Local banks.

I mean myself and any bank would need more details, but if you utilize that large sum as a downpayment and have a small mortgage, wouldn't you easily qualify for the debt to income?

Again, if there is a form of payment you do not accept, then I would line through it.All the forms can be edited.

Realistically, one checking account and an accounting system will meet all your needs and keep your business better organized than 7+ bank accounts.If you want to collect bank interest from excess cash, add a savings account.