Stereo "upgrade" for a non-EE

[Weetabix]

The rig: 2004 LC w/ nav, JBL w/ cassette deck, CD changer from console MIA.
Me: I like music, but I'm not an audiophile. I have no desire to deep dive into stereo installation and debugging. Ideal listening: radio, USB stick, Pandora, etc.
The issue: Suddenly, the speakers are just a whisper at peak volume
What I've done: I've pulled and reset all the harness plugs at radio, amp, and nav (? - whatever that other thing is under the passenger seat). Checked all fuses. The amp fuse is good, but I replaced it anyway. Problem persists.
What I want to do: Take it to an audio shop. Have them get what's there working again. Then replace speakers, upgrade amp. Maybe add the BT45-TOY later. I'm pretty sure I can live with a fully functional system that sounds OK and no CD changer.

My questions:
1. What should I ask for in terms of speakers, amp, etc?
2. What should I watch out for in terms of upselling me beyond my capability to enjoy?
3. What should I be asking you that I haven't thought of?

 

[Live to Ski]

I can only "speak" to speakers: the '04 (at least mine) has rare impedance values. Only a few modern replacements have that impedance.

Results (speculating here): lower max volume higher distortion?

Best and cheapest to refoam.

Extra credit: tighten window regulater bolts while in there. They loosen due to road vibration causing load "click" at end of raise/lower action.