What have you used for automotive body primer with good results? Everything I've used up to this point for engine and underneath has been Sherwin Williams Genesis line and their DTM sealer with good results. Moving onto the body, I believe I'd rather use a primer that can be sanded vs the epoxy sealer I have which I'm not sure would be thick enough.

Second, are the $500/gallon primers from PPG or Sherwin noticably better than cheaper stuff like TP Tools 3 in 1 at $100?

DP 40LF ( now lead Free) PPG and Evercoat Slick Sand

Upol 2253 is a great high build DTM primer that I've used on three cars with great results.

Im using the DP90LV as a base direct to metal etch, then using the PPG Vibrance VP2050 high build which is also a DTM, for extra insurance. Im then going to a "Impact" brand IMP4800 2K Urethane Primer and I so far really like it. Shoots nice, wet sands GREAT! I was using a NASON brand 2 K primer, but they had a issue at a manufacturing plant and I couldnt get any activator for it, so my paint rep told me to shoot this IMPACT brand. A super high end body shop here locally uses the impact sealer and told me it is by far some of the best he has laid down.

I guess there is a lot of options out there, and the way I see it is yes the materials are crazy expensive, but in the long run when my Daughters get my cars they wont have to worry about any paint issues, or rust coming from poor prep.

For good sanding qualities, you need a surfacer.
Polyester, 2k urethane, or hybrid are your material choices in modern surfacers.

For corrosion resistance, epoxy is a very good bet direct to metal which is why many of us start with an epoxy and then apply surfacer of some kind on top of it.

On the GTX, SPI epoxy direct to metal, then SPI 2k urethane surfacer for blocking purposes.
They have a standard and a high build, I used both but preferred the standard/it sanded nicer for me.
https://www.southernpolyurethanes.com/
They treated me right. Shipping was included on everything + the tech line is the owner's cell phone.
They also have their own forum like this one with a ton of information available.
I also used their clear coat.

Years ago used pre-Valspar Matrix epoxy + 2k urethane surfacer combination before with good results, but don't know if the product is the same now.

Next project I want to try a polyester surfacer after epoxy, as I think it will help me do the job faster.

I'm with Zippy on this one. SPI products work very well and are reasonably priced

Me 3! I've had great results from SPI epoxy primer top coated with their 2K High build.