Stripping and prepping an airplane for paint is a huge job. Painting requires skill and appropriate facilities.
If you want to invest some elbow grease, you can strip it (or soda blast it) yourself, using appropriate environmental controls and protective measures for yourself and the airplane. Then you can pull the plastic bits (cowl, wingtips, fairings, etc.) and sand and patch them. After that, you could probably get someone to paint it for a much lower price than $12k, then you could reassemble it under the supervision of an A&P.
Alternately, you can do a "scuff and shoot" where you paint over the old paint job. That's a halfarsed solution, IMO since it adds weight and also because you're never sure of how well that old layer of paint is attached.
As to automotive shops, how are you going to get the airplane there? Is it a model where the wings come off easily? Are you going to disassemble it that far? It can usually be done, but you'll need a lot of A&P support.