It requires a bit of elbow grease and time, but is not that difficult to do. My suggestions based on experience woodworking and with my 2 Sabres are:
1. Sand lightly with no coarser than 120 grit.
2. Make sur that you don't sand through any plywood (e.g., bulkheads).
3. For dirty areas, use acetone and a 3M green abrasive pad. It will remove the dirt and won't clog. They can be found at Lowes in the Paint section and look like a pot scrubby.
4. To fill deep holes, mix some mahagony dust (from a sander) with 5 minute epoxy. It will stay put and will match the surrounding area. Test in an inconspicuous area first. For light scratches, either do nothing because the finish will "erase" the scratch or snad until the scratch dissapears. Be careful of sanding through veneer though! For a test, wipe the area with a cloth dampened with paint thinner - it's an approximation of what urethane will look like
5. No real need to sand off the varnish unless it's discolored or peeling. Just sand lightly to remove the sheen and/or dirt/grease.
6. You can use any wiping urethane such as Formby's or Minwax. If you want a real deep finish, Minwax has a number of nice sheens and is readily available. No need for a "marine" varnish since it's interior. However if you're going for a deep finish (e.g., a table) try Minwax Helmsman's Varnish. It's really a urethane but is more flexible than a standard "varnish".
7. For floors, you can use Minwax Helmsman's Varnish. I've had good results with it. After about 3 years, you may need a new coat if it become scratched. But that will happen with any finish.
Good luck! It's worth it.
Sabre 38 "Victoria"