Up until lately I would have recommended PPG's DP line of epoxy....then I tried SPI and I must admit I've been converted. I would recommend the SPI for several reasons. SPI can be sanded whereas PPG's DP line does not have great sanding qualities. SPI has respectable build, so with 2 coats you would be getting a decent amount of mil thickness for blocking and straightening out a car. Plus it's got a great price point...It costs about 1/2 of what PPG's DP line costs. As far as quality of product...all I can say is I've tried to abuse it to failure (I mean I pushed it to the limits so if you follow instructions it will last a long time) and it has stood up extremely well.
As far as wax and grease remover goes, yes, use it...sanding will not take off old wax or grease or Armorall or any thing like that. In fact sanding will only grind whatever contaminant you want to get rid of into your existing substrate and give you potential for your new product going over top to fail.
Hope this helps.