There are a couple of club sponsors who are insurance brokers. I usually use a comparison site to get a base line, then call them and Brentacre/Adrian Flux to get a final price.
I'm not sure about the stock manifolds, but if you get really stuck, you can fit any 16v manifold to an ABF with a little work.
As for the letters at the end of the part numbers, they're usually for variants of the part. Usually start off with no letter, then go through the alphabet every time there is a revision. Technically, it should fit as long as the main numbers match.