From the research I've done it seems to me the best approach is to sandblast the keel white, then immediately apply a base layer of epoxy. Any recommendations as to what to use for the base layer? Is something like WEST systems alright, or is a zinc enriched epoxy necessary?
Sandblasting is correct - any grinding method will not clean all the pitted areas fully and future failures will start there. Just use regular epoxy resin as a seal coat - nothing fancy, just get it on right after blasting and a solvent wipe. You can't even leave this step overnight. West is only more expensive - twice as much as the industrial no-name resin I use. Stone fabricators - counter tops etc. use a lot of epoxy - check around and you'll save $hundreds.
Moving on to a filler, again is WEST ok? and how many coats is recommended?
Again, West only costs more. Get a cement bag of industrial talc for about $20. It makes the most beautifully sanding filler you have ever experienced. Mix it to peanut butter consistency with epoxy and you probably won't even have any pinholes to fill after sanding.
Many people claim to use a notched squeegee or trowel to apply the epoxy, and then fill in between the created lines? is this necessary?
This makes it much easier to get an even coat. Unless you're an experienced plasterer, using a plain trowel will give you a very uneven thickness. You do not want to sand through to metal, ever, or you will have to start over there. You want a reasonably thick finished coat - 1/8" min. is my preference - in order to ensure the metal stays buried.
Get an autobody longboard sander - pic attached - they look kind of like an old smoothing plane, about 18" long and take pre-cut strips of sandpaper. This will help you fair up your keel - mine ended up looking like it had been templated - before & after pics of the last one attached..
I finished mine off with 3 coats of epoxy resin and 3 coats of Interprotect (there was still discussion as to the best sealer at that time). Now I'd just use Interprotect for all coats.