OLD parts
On your old parts it depends on the condition of the paint. My 90 has the original paint and it chips real easy and has a bunch of chips that will need to be sanded so I am planning to strip, etch prime, seal and then paint
If the paint on your old parts is anything like the paint on my 1984 it comes off real easy just 1 coat of paint stripper and the paint fell off. For me stripping the old paint off is less work that trying to sand all of the little chips and ending up with old brittle paint under a fancy new top coat.
Test a small area and if the paint on the old parts comes off real easy then you might want to strip the paint off and get down to a good base metal.
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NEW parts
Sand (I use wet 400 or 600, just depends on the top coats and how I am applying) or scotch brite, whatever makes you happy. When I am sanding / prepping a new part my goal is to make sure I do not see any glossy paint when done, I want it nice and flat.
Your local paint store will have the data sheets for the etch primer, sealer and paint that will have basic directions such as use grit XXX wait x amount of hours and so on and so forth.
The product you choose will make a difference in prep so ask them and they will be able to tell you what needs to be done.
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PAINT
If you prep everything, buy the paint and spray it yourself chances are you will spend more money on paint and stuff to do it than you would have spent if you had prepped it and paid someone else to do decent / nice spray job.
If you want to go bottom dollar even a cheap place can lay down a decent coat of paint probably better than you could and it would be less than you are going to pay for paint and supplies if you do the prep.
Trust me, I have painted many cars and trucks and that stuff adds up quick if you are not in the biz ( I am not).
If you do decide to paint it yourself you need to spend some time at the local paint store talkiong about product and think about the following.
Are you going to rent a booth, paint it outside or paint it in your garage?
Do you have a dry source of air that can keep up with a spray gun?
Do you have a spray gun that can spray the different products that you are going to use?
I honestly think you should give some serious consideration to doing the prep work and having someone else spray it for you. Paint stores normally have a list of painters and if you tell them what you are looking for they can point you in the proper direction
for example: I am looking for someone to paint some car parts with a single stage enamel, I already have them removed from the car, sanded / stripped / whatever can recommend someone that will do a (chose quality here) at a decent price? I bet they give you at least a few names.