Hey Mike...welcome. There's no such thing as a dumb question.
You can replace that head unit with any brand you like and if it's not a full double din now, all you need to fit that size unit is a double din bezel. Lots of them on eBay and they are also available from your dealer at around twice the price as you'll find online.
I went from this:
To this:
Instead of replacing the amp, you can use it to send a digital signal to a new, more powerful amp. I installed my JL amp under the front passenger seat.
For aftermarket amps and head units, wiring harness adapters from PacAudio will be needed. And, if you have steering wheel controls, there's one more for that too.
Your speaker sizes are:
Dash - 3.5" (you may or may not have wiring up to this location depending on what system you have now)
Front doors - 6x9
Rear doors - 6.5"
You are not limited to these sizes. You can go round in the front doors and up to 4" in the dash, or smaller if you just want a set of tweeters up there. The best place to mount the tweeters, IMHO, is in that section of plastic in the front corner of the front door.
There are a lot of options and we haven't even considered the subwoofer and amp yet.
Good luck and if you have more questions, post up!