I'm not sure how you do the calculation and I don't now much about your house plan details. I had almost the same idea when I began my house construction. I thought it could be done with less than เธฟ500,000, but ended up to 700,000 for a 50sqm house. Perhaps my house is kind of luxury small house, but there are always too many small costs that will make a big difference between what we think and what it will become. Even though I did all plumbing, electrical, partly flooring, kitchen cabinets, sliding doors, push out windows etc,etc.
I built this house over a lot that belongs to my gf children and I'm always ready to walk away without regret๐. To me, 700,000 is 4 or 5 years rent where I like to live at. It wont kill me if I lose it and I'll be glad to leave it for my gf as an art work that I did ๐.
Do it if you can easily walk away without regret. Perhaps for you having a long lease paid all rent in advance would be the best choice, but keep in mind (I don't know your friends) there are different ways to make you move out and you will (for your own peace), even though you have a lease in your hand.