TOPIC: New to classic glass, need advice.

I need to rebuild the stringers, floor and transom on my 72 Century Arabian. Im a little confused as to what resin to use, epoxy? Polyester? Whats best?

Boat was built with polyester

You will l get lots of opinions on this but here is mine after 10 full glassic restorations over 20 years all being 95 plus points at ACBS shows when judged. Not bragging just so you know there is some experience. That said many very knowledgeable people hear with varying valid opinions based on experience.
I am a big believer of epoxy for many reasons. Structurally it has much better bonding properties than polyester. Epoxy sticks to cured polyester much better than polyester sticks to cured polyester, this is paramount at least in my mind. Epoxy is less permeable to water intrusion, polyester does absorb water,as evidenced by blister issues you may read about on polyester based laminates but I would consider this a secondary reason for epoxy.
I find the pot life is better controlled where you can get slow hardners for epoxy and mix ratios are straight forward also, they are low odor which is a benefit in my mind. Bonding strength, odor and pot life where the qualities I settled on to go the epoxy route. One thing is if you are regelcoating any patches done with epoxy it is more problematic, I have done it successfully and has held up fine 10 years later. Or you can possibly patch the outside using polyester based materials for the cosmetic top layer if gel coating then back up the inside with glass and epoxy for strength..... done that too.
The other thing is what glass laminate to use, I now almost go exclusive with glass fiber biaxial 45 degree stitched materials in wide cloth and tapes. Beware of getting this with a layer of stranded mat attached as epoxy does not breakdown the adhesive used to stick the mat to the biaxial, you can get this without that adhesive being used but you need to ask, I did a balsa core boat and having the stranded mat and biaxial together meant I could have a very resin rich layer of glass, the stranded side, bonding to the balsa, no balsa core in yours so no need to use this version. Raka in Florida is a great supplier of glass laminate materials. I also would recommend you get a material sample kit from fiberglast as it makes deciding what weight glass to get when you have samples in you hand.
I use West Epoxy that was pretty much all that was available in an easy to use dispensers back when I did my first resto, it does blush on cure, that needs to be taken off for subsequent layers for those to bond right to it. West has instructions on their site. I also rough up any new work after cure also to promote bonding. There are many new epoxy formulations out there now that do not blush I just use West due to a long familiarity.
Oh here are a few pics of my Arabian I restored a few years back, still have it , one of my favorites even though at heart I am an outboard guy.
A garage queen that still had issues due to poor build quality, all 100% now!
Good luck, there is lots I can tell you about in detail as to what you will find PM if you want with a phone number.
Anyways my 2 cents,
Randy