You have several options. A pool your size could easily get-by with a 1 HP pump. Your next decision is whether to try and find a 2-speed or go with some of today's variable speed models. In some parts of the country, a VSP is mandatory, while in others like mine you can still get 2-speeds. There are many more VSP options today than there were 5 years ago, but of course costs are going through the roof. I've seen some Calimar models rather affordable. The big names have VSPs, but you pay for them as well.
As for the filter, a 12K pool you also have many options. Sure, you could run with a single cartridge like I do in my 18K pool and save a few bucks, but you'll have to rinse it off more frequently (every 3 weeks or so) than with a large 4-cart filter. So it really comes down to convenience and cost. Just be sure to check your plumbing configurations and pad size to make sure you have the space and correct plumbing sizes for whatever make/model you chose. Good luck!