OP, a budget of 15K is extremely tight and in my opinion not doable unless you own a body shop and don't add the labor into the cost.
Is the car a recovered theft? If so, can anyone vouch for the engine and transmission?
You need to do your homework before buying this wreck. It could end up costing you more than you are willing to invest. Take the 15K and buy yourself a nice older 328, or if you want some Zip, buy an E46 M which might need a little work. You'd have a clean title on a sharp BMW which will only go up in value.