I'm guessing you probably won't be able to get a bank loan with those items missing so you might be stuck with them installing them.

You can do a subject to transaction but that is not legally assuming the loan.the banks got rid of assumable mortgage long time ago. 

In most states all you need is a separate account in a bank in that state.

Also remind them that they may be held responsible if a repair is necessary because they failed to change the filter.

When you are on the commercial side of the bank, and borrowing on the bank's balance sheet, they have a lot more flexibility to how they handle loans.  

@Seth RolandLoans like bank statement loans or DSCR (Debt Service Coverage Ratio) loans could be great options.

However by your own admission this is a large bank.

Your local credit union or bank should be able to help you with that 

My concern is that if i apply for a HELOC to use for the down payment before i apply for a mortgage, A) I wont get approved due to my DTI ratio, and B) I wont get approved without first showing the bank I have the 20% down payment before pulling the HELOC.

@Brandon Weis just an overly conservative bank. 3.5% down with FHA on up to 4 units.