If you're determined to do this from the bottom... (and it seems you are) then take some time to research vacuum bagging - it's do-able with a shopvac, the right film material and a bit of preparation. The guys at Fibertek can help you with that. You'll still have the difficulty of getting the core to stay put while you're placing it, but should be able to apply a uniform pressure once everything's in place. Or you may consider doing smaller sections, then trying to glass it over after all the core is in place. That'll add to the project time, of course.
Epoxy cure times can be slower than polyester, which will add to the difficulty.. but epoxy is the way to go - probably a thickened mixture for the initial coring application to limit sagging. Talk to someone about which hardener to use when you know what your working temp is likely to be.
I'd be tempted to enjoy the boat this summer, see how things go and maybe you'll have a different perspective by then - maybe even be ready to move up and decide to sell her.... someone will buy her at the right price (after all - you did!)
Oh - cost wise - doing all the work yourself, I'd guess materials will run you $200 for resin/hardener, $100 or more for the core material, plus brushes, squeegees, sandpaper, solvents, grinding discs, etc etc for likely another $100 easy. That's probably a pretty low overall estimate - these things always go more than what you'd expect.