Doors and Windows - Refinish old interior door?

I have an older interior door, 30" X 80". Painted on one side, stained and varnished on the other. After about 20yrs of dog scratches, grooves, and my son slamming it I need to refinish it. Is it better to use a chemical stripper to get the paint and stain off before sanding or just directly to sanding? Also, what can I use to fill in the grooves/scratches? I think some are too deep to sand level. Last question, the corner of the door, there is a chuck taken out; about 3/4" by 1/2" (from slamming) can I fill this with wood filler? It is on the narrow part of the door (the 1 3/8" side)
Thanks

I would just sand the door, roughing up all surfaces to accept the paint. As far as the gouges go, I would use Minwax wood filler. It is a two part epoxy that you mix in small quantities and apply it using a wide broad knife over the gouges. Allow it to dry and sand. You will have about 15 minutes of work time. Now as far as the chunk, not sure about that one, since I can't see it. If it broke out due to slamming, any repair you make will loosen by slamming, so you may not win on that one unless you correct where it was hitting. You may have success in using the epoxy.

For the varnish to paint well, you need to apply a solvent based primer after sanding.

I have had good results on broken corner repairs using either Bondo or the Minwax filler. The trick is driving an appropriate size finish nail (or nails) partially into the broken area to give the filler something to hang on.