A rebuilt engine is as reliable as the machine shop and builder. Ask around about whatever service you need. I build and have a trusted machine shop - working with them for some years for my road cars. I don't build modified, maybe a bit improved..
Good - Fast - Cheap: Pick any two.
E23 735i; E30 318i; E32 735iL; E38 735iL; R50 Cooper; R55 Cooper Clubman.
E21 318i; E32 735iL; E34 535i; E38 730iL; E53 4.4i