I have a 96 blazer that is currently dark green that doesn't have any rust on it so I would like to paint it dark blue I'm looking for good ideas on how to start and how much sanding do I have to do on the original coat. can I just buff It smooth then prime and paint? What types of paint would be the best to use? My next step would be to also paint the trim as you can see they're very faded. So pretty much not show had this car for 20 years like to do every little thing I can to make it look better.

 

Buffing is intended to bring the shine back to paint, not to help new paint adhere. If your paint is not peeling or entirely sun faded, buffing is a great option to bring life back to dull paint.

If you are dead set on repainting it yourself, Sand it with 600 or 800 grit, use a sealer(I use an epoxy sealer) and use urethane paint. However, If you actually want it to come out looking good I would highly recommend finding a reputable body shop in your area and having them do it. It's worth paying the price for a proper quality job