Pretty typical construction for the era & good news on both fronts.
You'll need to make the inner/outer skin determination for yourself, but either choice is fine. Each has advantages & disadvantages. Weather windows have some influence, too.
I really like Gougeon Bros.' West Systems epoxy and use it for critical applications, because I explicitly trust their standards of manufacture. But for bulk work like this, you can save 50% by purchasing discount epoxy (I wouldn't even consider polyester resin for this job.) There are a number of formulators out there -- and that's all they are, because everyone buys their resins and hardeners from the same two sources. They dilute the hardeners and sometimes add solvents to adjust cure rate, flow out, and mix ratios.
: Some of these discounters supply (or sell) pumps for their epoxies that are not metered
. That is, they don't dispense the correct amounts of resin & hardener on a one-pump to one-pump basis. Even if they are different colors, or have different numbers on them, the pumps are identical.
YOU have to count the right number of strokes, like "Three resin, one hardener." If you are used to metered pumps from top-end vendors like West or MAS, there is room for heartbreak here.
Do not ask how I know this.