@Ashley Nelson always a sweet deal when you can secure with seller financing! It's hard to walk away from that when available. In your example here I really see two options:
1) Keep the seller financing, and utilize an unsecured loan by means of either a private investor (whatever you work out between you is your business), or by that of a personal loan, which would require very good income, credit, DTI etc to hit the $400k amount you're looking for. If you have an operating business with strong monthly cashflow, you could also look at an unsecured business loan.
2) Utilize a Purchase and Rehab loan with private/hard money. Depending on how well the numbers pencil out, location, assets, your experience etc, you could potentially obtain around 85% LTC. Some will also let you leverage a seller 2nd for the remaining 15%, though you'd still need to show 20% of rehab in reserves + sometimes 6 mos interest-only payments in reserves as well + closing costs. All things to consider.
Not all lenders allow for seller 2nds of course, so you'd have to shop around for one that would allow it. Im not sure exactly where this deal is located.
Thanks and hope this helps!