I've decided I want to paint my car myself. I'm not going for some award-winning concourse-quality car with a $10,000 paint job, just something that looks better than the beater I currently have. The car was in an accident long ago and repaired well, but it's not 100% perfect, so it doesn't make sense to me to spend tons of money on it. I'd rather drive it and enjoy it. That said, it's still a nice car and runs great and looks good.

I plan to spend most of my time finishing the parts of the car that will show and less time on the parts that are usually hidden: under the hoods, the door jambs, inside the trunks, etc.

What's the minimum I would need to do to get these surfaces ready for paint? Scuff with Scotch-Brite? Sand with 220 or some other grit? Sand blast? Is it better to use power tools or do it by hand?

The car was originally metallic silver. It is currently white (engine bay, area by fuel tank are still silver). I plan to paint it a solid (not metallic) gray that closely matches the silver.

Don't aim low, it is not difficult to use a two stage water based silver metailc and get good results. That with an SPI clear will produce great results.
Minimum is a scotch bright red scuff for most parts and jambs.
Use a good epoxy primer as your base for filler, again the hot rod guys swear by the SPI 2 part epoxy, not an ad just an endorsement.

Wash with soap and water. TSP works really well, but it will take the finish off paint you might not want to get damaged, but I like it. Get everything really clean with soap and water first.

Wipe everything with Prepsol, Klix88 or some kind of cleaner, wax remover solvent. Apply solvent with clean rag, wipe off with clean rags, and change wipe rags so you aren't spreading around contaminants.

If you don't get all the grease, oil, wax, silicones, and contaminates off the surfaces you'll have paint adhesion problems. You can't over clean.

A lot of shops use red scotch brite pads for inside the trunks and difficult areas to prep for paint. It's worked good for me.

Wipe off sanded areas and paint with the same solvent and more clean rags.

Epoxy primer sticks really well to properly prepared surfaces, but you want to top coat it within the recommended time window.

Thanks for the tips.

How scuffed does the finish have to look after using the Sctochbrite red? I have some and have used it, but I want to make sure I'm using it correctly.

Here is more info that may help:

I will be using single stage, solvent-based paint.

I was planning to just sand and prime with automotive primer in a can (Transtar 4603), then go over the primer with the final coats.

The local auto paint place recommended Linco brand paint. They also sell R-M, brand but it's 3X the price. A gallon of a solid color on Linco paint is about $130. Reducer is $53/qt and hardener is also $53/qt.

Keep your sanding to a finer paper otherwise you will see marks unless you use a fair amount of primer,I prefer 320 drylube around these area's as long as you are keen to do it right.
When you are ready to prime clean and clean again with paint prep.
Pick a simple paint option ,PPG is a great product,if you have no real experience,lay a basecoat on and get skills with your gun,you will learn when the gun is laying a smooth finish or orange peel.
If not happy with the basecoat knock it back with wet & dry (400 grit),get it right with a good prep wash then lay on good thick coats of clear...I do 4 coats minimum on a small car.
You mentioned a flat color that's fine so would I stick with white SO easy to lay on and looks real clean with clear buffed ,up to you.
However you go with your paint job it will be what you put into it ,a really great paint job from pro's is about prep then the rest is basic enough ,the rest is in the final buff.
Go well

No gloss, and flat is how far you want to go.

There is a tube of scuffing compound that comes out like tooth paste and is applied with red scotch brite.. It is a norm at all paint supply houses. Works really well.

Make sure that the primer you are using is compatible with the final paint. don't skimp on the primer, remember every coat on top of the primer depends on how well the primer sticks.

As others have said clean it as well as you can. use wax and grease remover and change rags frequently. maybe a couple fisheyes under the gas tank won't matter much but its good to get in the habit for when you paint the more important parts.

On my car while it waits to be worked on. On Doug's car, it was sprayed on by the blaster after the car was stripped.