Had not considered the intake manifold - oh dear another job. Did you consider the exhaust manifold? Has to be sandblasted first to gat all the surface rust off. Then 2 coats of 'heat proof cast iron'.
It lasts about 2-3yrs: or has on my E23. More manifold to see on the E30 though.
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