All the advice these guys have given is pretty dead on. Bigger is not always better.
A pair of 10s, pair of 12s, or a solo 15 should be about all you'd want in a daily driver. if you're building a competition buggy, go crazy with the 18s and mega amps.
I'm 33 now, and started into the car audio scene back when it was cool to put a pair of tower speakers from your house in the trunk. (Yup, I'm that old. LOL Anyone else remember those days? lol) I have had various systems over the years, and my favorite system in my daily driver was in my 96 Stratus. It consisted of:
Infinity Kappa 6" component set with 1" tweeters up front and 6" midbass in the rear deck, all powered by a 4ch Rockford amp (forget the model off the top of my head)
and for bass I had 3 JL 10w6 (back when they were new. dang, I am old) powered by a RF Punch 100ix2 (last of the pillbug style amps)
I wasn't running godly amounts of power and didn't have enormous speakers, but it was plenty loud enough for a daily driven vehicle. It was clean and clear and could be heard for blocks if I cranked it.
Another aspect to look at is vibrations. I hated some of the systems you'd hear around town. When the car/truck/suv would drive past and all you heard was Brrraaannng Brraaannggg from the metal in the vehicle rattling apart. I'd say take your time before you install the system and sound dampen anything that rattles, and go over the odds and ends and fill any dead space with expanding household foam to keep rattles to a minimum. Places like the rear hatch, roof panel, plastic around the cargo area, they're all going to shake like crazy no matter what you put in for a system.
Also, water damage from leaking window seals and hatch seals can become an issue if you run too much SPL in a daily driver. my 90 Escort with 2 HiFonics Zues 15s leaked like a siv. The rear hatch leaked as well as all the rear glass. Every time I'd reseal it, it would blow apart again. Eventually all the water that leaked in ruined my sub box.
Just some food for though... Let us know what you end up with.