Burton Power is a great supplier, especially for go-fast part but it might be easier dealing with someone on this side of the pond. Dave Bean Engineering should have everything you need and they carry go-fast parts too if you want them. r.d. enterprises is also a good supplier though, I suspect they concentrate more on stock parts. I've bought stuff from all of them and they all give great service.
When you rebuild an engine, you get a gasket set, but usually bearings and rings are purchased after a trip to your machinist and you find out if your crank needs to be ground or you want/need larger pistons.
Sometimes (always?) a gasket set will not include a head gasket since that is another variable. Head gaskets come in different thicknesses and materials.
A nice thing about the Kent block is that it can be align bored (making the mains closer to being in line and thus reducing some of the friction) without moving the centerline. When this is done, you order your main bearings by ID and OD.
edit: added a bit about head gaskets
