Welcome, I am at work and might address some of your questions later, when time permits, but buy then, others will have done the job for me!
Again, welcome - you did good on the price, if a DEALER sold it to you at that price, they either sat on the car for too long or they gave the previous owner NOTHING for it!

For now:
Spoiler motor failures (that are the MOTOR) are rare.
Sounds like you have a tire pressure monitor sending unit gone bad.
Most of your audio questions are easily answered: Replace the factory head unit with something modern and "better".
Body parts can be a problem, but most other parts are available. I ordered an EGR valve and it came in the next day - the GASKET will be here in a week.

Again, welcome and others will chime in with better answers than mine... off to work.