For your first question, most any name brand paint mfr. PPG, Dupont, Sikkens, RM, BASF, "should be" able to formulate a single stage urethane product, if they are still maintaining a single stage product line. Some Mfrs. do, some don't. The listed companies are all good quality products as long as you stick with the same line of thinner,catalyst,primers etc. I don't recommend cross mixing Mfrs products. Urethanes especially black would benefit from adding top coat clear to the last spray coats of the undercarriage and engine compartment. Urethanes also can be air dried which is what I like best. For your second question, paint problems arise from poor prep. If the surface wasn't sanded properly, de-greased properly, or not primed, the paint will have problems down the road. In addition,..having too much paint sprayed on will cause cracking. There are many gremlins that surface with poor paint prep. I just listed one of them.
The last point I'd like to make is, trust me on this...it will depend on your overall project budget. Purchase the quality of paint you can afford and live with.
To give you an idea of how much top quality Urethane costs, 1 gallon of PPG DCC concept single stage Urethane in V2 was over $600.00. Plus the cost of hardener catalyst, thinner, primer and primer catalyst, mixing cups, sandpaper etc., now your looking close to $1K. I know cause I've just completed my undercarriage, interior floor pans, trunk, and front inner fenders and frame rails 2 weeks ago. It looks gorgeous and the paint flowed out nicely and covered really nice.
I was very happy with the results. I have about a quart of paint left for other parts of the car to spray in the Springtime, so for me buying a gallon is what I had to do. Hope that brings the reality of Urethanes to your inquiries for under body and engine compartment area's. I would save base / clear coat paint systems for the exterior body panels. And don't forget to use a high quality full face respirator when using Urethanes, including a zip up paint suit, gloves, and eye ware even when painting in a home garage or outside.
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