Just like there are rockets that bridge the gap between LPR & MPR (like the forementioned Estes Executioner), there are plenty of rockets that can handle MPR & HPR motors. Kits like the PML Phobos, which I built stock & flew a couple of times on G35s before attempting my L1 with a Pro38 H153. According to
the reviews & flight log on EMRR, the Phobos (if built
lightly) can handle some Fs all the way to a J350 (with an extended 38mm MMT & fiberglassed airframe).
There are some folks who will fiberglass darn near every rocket in their fleet regardless of size (yeah, even their LPRs!) Me, I'm in no hurry to use that much epoxy & do that much sanding! haha There are even plenty of large LPR rockets that don't require fiberglassing...
Epoxy on the other hand is reasonably necessary for assembling most MPRs & all HPRs. While it seems daunting to have to mix & apply it, it's really not that hard. Until you get the hang of mixing epoxy & the actual amount of time you have to work with it, I'd recommend starting with 30-minute epoxy. 12- or 15-minute might be OK (I started with 12) but I've learned since getting my hands on some of the longer curing epoxy that my work comes out better when I have more time to work with it & I'm more relaxed when using it. That's a personal choice though...
The last change, balsa -> plywood, usually isn't a choice with kits... The wood you get is the wood you get. Again as a personal choice, I prefer working with plywood because it's much harder, more resilient to impact, & not nearly as porous as balsa.
Lastly, I'd recommend to start small & on scrap pieces to get the hang of these new materials/adhesives/techniques. It'll save you headaches & money. Good luck & keep us posted of your decisions & progress!