Aftermarket coils are cheap but quality can be dubious....would stick to German produced parts or OE if possible.
not sure of exact costs but could look on Eurocarparts as they offer huge discounts periodically
They are generally good engines and respond to routine maintenance with reliability.
Plugs that are gapped correctly, oil that is changed regularly, good quality fuel are just a few things to assist in avoiding expensive repairs
Hi Ben....welcome to the Club
Would be great to see a piccy or two of the M3....certainly a great car from a great era
Good to have you onboard
Cheers, Trevor