You may have been told this already, but I'm not about to scroll through all the comments on every page to find out. It would make sense for those back speakers to be 4x10s, that would be the same as my suburban. I would recommend converting the plates for those to 6x9 size. You can get much better sounding 6x9s than 4x10s.
If you don't care about your speakers being super loud, it should be fine to just power them from the head unit. That will likely push about 50 watts rms, which isn't bad. Plus it's a whole lot more wiring to get an amp set up.
For subs I have a single 12" Skar Audio DDX. It sounds great, and will take a beating. However, going cheap I would just look for a decent name brand sub on marketplace, you can probably get your set up for like $150 and then wiring for like $50. Don't make the same mistake as me trying to get a cheap amazon $30 wiring kit. The speaker wire melted and the fuse was only rated for 60 amps, so I had to spend like $50 anyways, and my power and ground wires definitely aren't as nice as if I just got a skar wiring kit.
I don't really know if you've had a lot of experience wiring this stuff up, but it's a good amount of work to get a sub in there, especially the first time. Take it step by step, you might not get it all done the first run through.