Depends on how much thump you want. I take it you just want a casual, straight system, nothing too fancy?
I've got the VU ute so you have to ask yourself, do you want to sacrifice some or all of the rear space behind the seats?
NExt thing to do, you got 2x 4ch amps now, with the more powerful one (or if they're the same...) use it for your speakers and perhaps sell the other one to put money towards a D class amp. Perhaps you could use both 4ch amps by running one on your speakers and if you want 2 subs, you can bridge the second amp.
Also with the subs, getting one or two can be plainly down to personal preference. Some people say 1x 15" sub is better than 2x 10" subs and some people are vice versa but it depends on quality and again, preference to your hearing.
Personally, my ute has an all Fusion system. I have 6" splits for the front, 6x9"s in the rear and one 12" sub behind the passenger seat and the amps (1x 4ch and 1x D-Class) and capacitor are behind my seat. Set me back more than say a Pioneer or Kenwood equivilent but I like the brand and the quality is great for me. If I move onto another car in the next few years, I'd like to go back to Alpine or I might stay with Fusion depending on who's got better quality.
Your main concern though is to set a budget and stick to it when it comes to buying time.
Research is invaluable and I don't know who said it but knowledge is power. The more you have it, the more likely you'll end up with something you'll like and enjoy.