I have seen a little bit of everything over the years.
if your paint isnt flaking, or peeling it should be a good base.
a good scuffing with decent sandpaper, depending on what your planning to use for a top coat. with house paints you can probably get away with 220 or so grit, but if you spray automotive paint 360 grit for some enamals, & 400 for most others so you dont see sanding scratches.
I have seen vans painted with spray cans, & 1 that was even clear coated with spray & it looked great. but it was primed, sanded, several coats of paint applied, then cured & sanded, & then clear coated. but I think he said something like 40+ cans of color.
when I was in college a classmate painted cars every week, he bought a used car from auction, applied bondo, & sprayed it with a wagner electric sprayer. they didnt look too bad, & he sold the car the following week at auction. he paid his tuition, & rent, gas money, & food for him & gf for 2 years doing this.I wasnt impressed with the spray, but it was ok.
I have seen cars painted with a paint roller, as well as bed liner sprayed. it depends on what you want, as well as what you can afford.
I made my living for many years doing body work professionally. & highly recomend using automotive paints. along with an air compressor, & a spray gun. however for a beater I understand the economics of using house or outdoor paint.
I have seen quarts of Rustoleum lately for for $3 per quart, while cheap cheap cheap car paint is at the least $60 a gallon. & good base coat clear coat can cost you $300 easily in materials, some lower quality base coat clear coat can be at least $150 ish.
I will do my best to answer painting questions.
Randy