I have used Epiphanes Bootlak for many years and it has worked well for me. It cost$.
It is a traditional varnish, which means that it is really a liquid plastic. You put it on as thick as you can without runs, and lightly block sand between coats. As far as I can tell, it is permanent inside and good for 18 months or so on deck in Florida sun.
Scratches and dings can be repaired by putting a little solvent (highly refined turpentine) on your finger and just rubbing it out, since it dissolves the varnish. If you use a catalyzed modern varnish, you can't do that.
Don't varnish inside the boat. Remove the pieces and work on them flat on sawhorses in a dust-free place. The trim comes off easily, and most of the panels come out without problem. Be sure to install everything that goes behind the panel while it is out: only do it once. I know this sounds obvious, but I had my whole boat apart for varnish, put it together, then apart for wiring, then together, then apart for insulation. Don't be a Doug.
I stripped my trim with paint stripper, filled dings and old screwholes with West Epoxy and phenolic microballoons, sanded it with 100 & 220, bleached it with Clorox, and then sanded with 220 and put on 4 coats of varnish. It still looks great 5 years later, and should for many years to come.
The original trim and ply in both of my boats was all mahogany. Yours could well be the same. Listen to Sorrydog (he been there) but try to save as much mahogany as you can -- that's what sells the boat when the day finally comes....
1975 32' Flybridge Sedan, twin Perkins 6-354 diesels, 1:1.53 velvetdrives, 16 X 19 props. Merritt Island, Florida