Sounds a good result Ian....do you have a photo of the print out that you could post up here...would be interested to see what areas were changed/improved.
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