restoration as a whole on a 35 year old vehicle? start with steering and brakes. if you mean specifically the paint, I'd leave well enough along especially if you plan on painting in the future.
One thing to consider, if the truck is a keeper but a driver, paint it yourself. I priced out local paint shops and it was cheaper to buy an 80 gallon compressor, run lines in my shop, paint gun and paint than pay for a low end paint job. If you have the space, it's worth the investment and a few test sprays and for a non-show quality paint job, a DIY works fine.
Will agree with above, a DA palm sander followed by a polyeurythane epoxy primer, would be the way I'd go. SPI (southern polyeurythane) makes a great epoxy primer that will essentially seal your entire truck and any remaining poor quality paint that remains on your sheet metal. as long as the paint is not peeling, you don't necessarily need to go to bare metal, thus saving the galvanized layer of your body panels. After the epoxy, you do filler work, reseal, 2 k primer and then paint.