Seller finance deal on house + buildable lots

[Christopher Juntura]

I'm negotiating a seller finance deal with a 4/1 house that will rent from 2500 to 3000. Along with it two .6 ac lots.

With the codes I can build 2 units and an ADU on each lot.

the numbers....

5 year term

6.5%

interest only payments

total cost $744k.

down payment (still negotiating)

house cost 450k

estimated cost to build 605,800

interest monthly 4030

potential rent income after construction $13500

taxes are 21.37 per 1k valuation

roughly 1,080,000 

roughly 1923 a month

Usually id leverage the land as collateral to secure a construction loan. At this point I don't think I can do that. Trying to make sense of the deal but I'd like to hear your thoughts on the deal and how to make it work or if I should walk away.