OK, i'll chime in here with my two cents worth. To strip or not to strip? On 95% of what i paint i do not strip it beforehand. Why? Because the factory paint is usually sound and stable.
i do, however remove all lettering and numbers as suggested above. i use 1200, 2400, or 3000 grit wet/dry sandpaper (available at NAPA or other auto parts stores) and sometimes 3M green scrubbers. Start with the finest and work to a more coarse grit as needed so not to damage the underlying paint any more than necessary.
Primer - yes, but don't use automotive or household primers found in rattle cans. Assuming your not using an airbrush please use a hobby primer in a rattle can (Testors - about $7 or Tamiya - about $10). The reason for this the pigments are more finely ground and the solvents and drying agents are formulated to the pigments and have less chance of damaging model work; this does a better job of preserving detail.
I agree that most auto and home primers are NOT good for models. They are made to fill in and thus not good for details.
Dupli-color sealer primer is not like this. It is a sealer not a filler. I have some expensive primers including Tamiya. Side by side I still go with the Dupli-color sealer. I pay $3 for 12 oz Dupli-color.....it could be $15 a can and I'd still pick it over Tamiya (which is near $15 for the small can now) There is no ONE perfect method. I think it's best people find what works for them......that is the best method for them.
All the above have two coats of Dupli-color sealer on them. Detail is still not obscured....