Sell it. A 15" frame wont fit you.
And there’s no ‘cheap’ way to build it up either…you’ll spend a few hundred bucks even if you’re VERY thrifty.
according to the specs, it has a 28.98 standover height and 22.8 top tube length. Coming from the bmx background, it might work out…
Parts CAN come inexpensive (not cheap, know the difference!) but it’ll take a LOT of searching and patience. getting the wheels and fork will be your biggest cost.