I think you're approaching this right, and having your own experience with a similar front stage without sub, you may be OK with a small one.

Perhaps installing the front stage with the 4 channel amp and then play around with different subs. I find lots of interesting options used locally.

I ended up mounting the JL 8 on the rear of the one seat in the hatch with L brackets and industrial velcro. That was a compromise I was willing to make, vs paying to have the stock sub removed and custom up a box in that hatch corner.