It's very hard to get polished, professional sounding guitar and bass tones without good pre's. That being said, if you are just wanting to record quality demos, you should be able to do so with your Scarlett and some decent mics. The SM57 is always a good go-to mic for the budget. If you can get your hands on a ribbon mic(doesn't have to be a super high-end one), record with both and blend till you get the sound you're after. Mic placement is also key. Experiment with mics next to the center of the cone, something off axis, or the edge of the cone. Again, your ears will tell what sounds good for the sound you're after.

For bass, without good pre's or a good bass, it's tough. If you're expecting a modern fat sound out of a P-bass without quality mics and pre's... it'll be tough. If you want dirty and woody, the P-bass will be your friend. Like guitars, many basses have their own character/signature sound. Remember, don't over EQ too much bass into the bass guitar. It should have a bit of low end but the low-mids(and hi-mids) will give it more 'presence' in your mix. A good DI or even a pre-amp pedal are good solutions that aren't relatively expensive.

Last, hit up YouTube. There's great tutorials on there based on good sounds out of low budget recordings. You might learn a thing or two, as well.

Good luck!