What would be the range of price for a single house around 60-80 sqwah, not new ?

I am not married with my thai partner (we have a son though) but is there any chance to get a mortgage , with guarantee on the house?

What is the best way to find opportunities, we tried a few times nakornthong properties co, they have quite a long list of houses but not to our taste so far.

Appreciate any advise

Not directly to the point, because I really don't know prices in Sukhumvit 71, but the continuation of Soi 71 is Ramkhamhaeng and many Moo Baans are being built there, and also some condos. You might want to check out personally. Ramkhamhaeng has become a real boom town place lately, perhaps because Suvarnaphum airport is close. So expect the prices to go up. Could be a good investment to buy there.

Expect anything between 1.5 to 3.5 Million Bath, depending on size, location, quality etc.