Will temperature really have that great an effect? And I've heard Talc has moisture retention properties, any truth to this?
Temperature spec's from the manufacturer should be followed - they vary. There are epoxies that are specially formulated for cold or damp or even underwater applications. They are generally VERY expensive though. I have found you're generally O/K down to about 50 with a fast hardener although it will slow down a bunch.
Talc does have moisture retention properties. This can be a very minor, occasional problem when using polyester body filler and especially with auto body primer over bare metal, if left unpainted - that's where the reputation came from. When mixed with epoxy I've never heard of it. I was given the process by an old time boatbuilder and boatyard owner - it's always worked superbly for me. Don't forget, worst possible case, even if it acted like a sponge, you'll still have the epoxy resin coating under it and several coats of barrier coat over it.
After sandblasting is one priming coat of epoxy sufficient on the keel, and then i can move onto filling?
Yes, strictly speaking it's not a primer coat, it's a sealer coat - you get it on fast to seal the bright, unoxidized metal from the air to prevent any oxidation which would degrade the bond of the coatings.
How much dry time is necessary between coats in your experience?
Until it's hard enough to not gum up sandpaper - varies depending on the catalyst, air temp etc. Sometimes the starting area is cured enough by the time you've finished the end. - It's very easy, you'll see. If you've ever patched or mudded drywall you'll have no problem - very similar process.