I think a 203b loan is for new or existing. If you want to build, you need a construction-to-permanent-financing loan. You could call Washington Federal to see what they think about an owner-builder loan.
It looks like you might qualify for a total house payment around $1,000/month (unless you don't have ANY other scheduled monthly debt that shows up on a credit report). Total debt-to-income ratio for approval is 41-42%, typically. So, the house you buy or after-built price, depending on whether you are in a high property tax area, would be $125-150,000. That cost would include the lot, the city/county's systems development charges, plans, etc., and be a finished, move-in-ready home.
You might also want to contact Highline Homes, Adair Homes, or Reality Homes for package, stick-built homes.
There may be a 680 score limit on some lending programs, as well.
I also have a bank that has money available to help with loan down payment, closing costs, prepaids, etc.
Let me know if you need more information. Call me anytime 503-580-2941.