You've got choices about where to get/buy all your stuff.
Ken can give you the scoop on tax/import. I've heard from several folks that somehow the stuff ends up being taxed. Because of that many people do drive stuff across the border. If you've got enough lead time, you could get in touch with Downwind Marine in San Diego, CA. They not only ship things all over the world, they also "send" things down into Mexico with cruisers who are headed south. It works if you're in PV or Mazatlan or on Baja. Not so sure about San Carlos. That major migration of cruisers will happen in the fall, not now. If you're in the mood for laid back adventure, you could buy some stuff in the USA (California) and crew for one of the Baja Haha boats that might have room for your stuff aboard. There will be many, many boats going South with the Haha and some of them will be very inexperienced skippers who would love to have an experienced sailor along. Food for thought.
San Carlos--lots of folks have worked on boats there. CL member Sea Venture has done that (however they're on the east coast now). CL member Dobrodaddy's boat is on the hard there right now at Guaymas while he's visiting the USA.
People rent cars in Guaymas and take the rental up to the USA to buy stuff and bring the stuff back down to their boat. We have a cruiser we've run into here in San Francisco that did that--I'll track them down in the next week and try to figure out how it worked out for them. We've personally driven through Nogales but that is one of the few places where there are drugs and violence so you won't tarry there for sure. When you take stuff across the border, as long as it looks like something you might use on vacation in Mexico and it doesn't look brand spanking new they don't ask you questions about it. Always in our own car we've taken huge quantities of stuff across at Nogales, Mexicali, and Tijuana.
Police--they've always been nice to us. So, ?? I guess so.
You're going to enjoy yourself