I realize your question is directed to Monmix, but I'll try and give you a "pre-reply" reply.
First off the best primer is factory paint. So unless it is totally bad for some reason there is no reason to try and clean it all off.... plus the cost of doing so is high. So to answer number one, no - no sandblasting.
What you will want to do it to sand down to a stable useable substrate paint...
Now, in the old days you often shot a coat of sealer to prevent the interaction between the old paint and the new. With factory urethanes these days, that is not so important.... but... if you DO fill rock chips, you will want to prime over the filler.
Laquer paint is all but a thing of the past. Enamels have evolved into urethane enamels, and decent single stage (eg. one coat) paint is now mostly all urethane enamel. Basecoat/clear coat is a different animal....
Base/clear has a base that you apply and allow to dry into a certain "time window"... at which time you need to top coat with the clear. They chemically bond to form a single layer if it all works correctly. The base dries matte.... the gloss all comes from the clear.
Urethanes are expensive...and the ultimate price is dependent on color as much as anything.... plus there is the cost of the clear.
In base/clear, you usually shoot base to get good coverage, and then add at least 2 coats of clear. I like to do a fog coat followed by two full coats of clear.
In metallic, I have done a clear coat, followed by a mix of clear mixed with the metallic and then followed by a clear. I expected great depth. Eh. Probably not worth the risk of playing chemist. Paint today is all about chemistry. (Which is why in the 60-70's a car looked like shjt after about 6 years and today a 10 year old paint job looks so good)