Professional painters generally don't prime over existing paint. I myself do painting professionally but I only do it now and then. I'm basically a handyman and I manage rental properties and have done complete remodel jobs on rooms from floor to ceiling and can also do a lot of electrical wiring.
As long as you buy a good quality paint you should be good to go. I would suggest buy the best quality paint you can afford and buy one with primer added into the paint. I would only buff out (sand) high gloss paint. So anything else (semigloss, eggshell, satin and matte) i'd paint over.
I take it you're painting the exterior of your home? If that's the case, it's always a good idea to power-wash every surface that is going to be painted. Regardless, primer or good quality paint isn't going to stick well if the surface isn't squeaky clean. You can rent power washers from Home Depot or look for a tool rental hardware store in your area. If you've already painted don't fret about not having power washed. But the good thing about power washing is that the strong force of the water from the washing will rip off any loose old paint that would otherwise not be a good surface to apply new fresh paint.
Good luck. Hope it's going well.