Alternatives to oil if you're interested
I've used Sherwin Williams Pro-Classic Acrylic enamel on a built-in and it's proven to be very durable.
For me, at 80 grit, my old random orbit sander leaves a pretty smooth surface that I'd be happy with for most paint, but that's a random orbit sander, not a belt sander. For varnish or lacquer I'd go finer. I think it also depends on what sheen you're planning to use. Gloss shows every imperfection, but semi-gloss and satin are more forgiving, especially if you use a good primer. If you're going for flawless gloss, then you're into primer, sanding between coats and that kind of stuff to get the mirror top coat.
The purists are going to hate this, but you can get away with a little bit more imperfection on the baseboard since it's not at eye level. Also, most people aren't looking at trim up close with a magnifying glass.