Real estate opportunity or bust?

I'm a relatively experienced contractor looking to buy a defunct 1800s historic (but not historically registered) single family home. It's a total gut job. Houses in the immediate surrounding area have a $415k median sale price in about 30 days listing. The house is just over $150k. The lot is multiuse zoned.

I have $45k in accessible cash. The seller is offering seller financing.

I'm thinking of offering around $125k because of the need for full remodel on the house. To the studs. New electrical and plumbing. Offer seller financing of 6% balloon loan and using my home equity ($350k) to pay for repairs, of which I'm estimating will cost $100k-$150k.

Am I too busy second guessing myself?