Not sure what market you are in, but I would talk to some lenders about FHA options on a 2-4 unit property.

While you work on that, you could also explore creative financing options like DSCR loans or partnerships to fast-track your first STR.

Has anybody found a workaround for this or would the seller simply have to hold the note long-term without the option of selling it?

What ever the Contractor proposes you should not take as the only option.

build to rent is great. they are an intermediary that help investors contract direct with turn key new build. they are pretty sophisticated we have sold a house so far and we are looking to sell other multifamily with them. there is a vetting process and as an intermediary they don't control construction they help vet builders and establish relationships. they have a big team. the other builder I've never heard of but unless you can see your house I wouldn't touch it. rent to retirement has a strict policy of 90 days until certificate of occupancy until they will send a deal out they are good at what they do and I'd recommend them. they have options all over the country and established relationships.

You have options!

One long-shot option is to offer a lease at something like $1,300. 

I have been able to find a couple of options for this setup though, they just require separate leases for each room and sometimes 2-3 months of rental receipts as well. 

Happy to chat and help you compare your options.

As others have suggested, you could get a conventional or FHA loan if you want lower down payment options.