There aren't any cheap 5 lug conversions. It's $1000 minimum and usually $1200 to $1500.
On the rear you can use M3 hubs. They're 5X120 bolt pattern. On the front only the 4 lug bearing/hubs fit. You can use M3 struts which are very hard to find used and pricey new. Or you can use the E36 struts. They're cheap used. They do put the wheel to the rear of the wheel opening a bit.
4 lugs are more than adequate from a performance standpoint and are not a weak link even in competition. BBK's and wheels are also readily available. There are 7 types of new BBS available in 15, 16, and 17 inch. There are lots of used 4 lug wheels available also.
I have never really seen the thing for 5 lug conversions. You can buy 4 hot new BBS for the price of the conversion. The 5 lug setup also takes you out of the spec competition classes.