Don’t belt sand old ledge doors, you’ll ruin them! Unless you want to remove all the character, in which case you’re best buying some new ones.
Chemical stripping is the best way, I’ve got one to do myself. It takes a fair few goes, especially if it’s old lead based stuff (which it’s also not a very good idea to sand because of the dust)
Having said that, if you ignore all the above and do want a belt sander, Screwfixes ‘titan’ ones are actually quite good for the money. Mine was about 50 quid and did a sterling job of renovating a ruined stair case. I reckon at least 50 hrs work. It’s also helped in loads of other stuff like building a wardrobe, sanding small bits of floor etc.