A Few Stereo Questions... Amp and AUX button

I am trying to decide if I want to replace the stereo in my new-to-me 2001 Tahoe. The CD player does not play mp3 cds nor does it play CD-R's, nor does it sound very good. In a perfect world, I would like to take my stock stereo out and replace it with a head unit that had bluetooth/hands free options, an aux plug (or 2). I would be disappointed to lose the tape deck though because I do occasionally use tapes when I go on road trips. I am a bit of a book-on-tape fiend. I am assuming there are ways to remove this as well - or to buy a piece of fascia that covers it up.

Also I noticed that my truck has an AUX button on the head unit. What does that mean exactly? Is there a set of AUX rca plugs hidden somewhere that may give me some more options I hadn't previously considered. I would also like to put a subwoofer and subwoofer amp in this vehicle so that would mean I'd need preamp outputs from the stock stereo.

Just looking for some of your thoughts on this. To sum up my questions which maybe should have been broken up into individual threads...

1. What is the AUX button for on the stock stereo?
2. Does the stock stereo have preamp outputs?
3. Where do you recommend I install my small subwoofer amplifier
4. What head units have you had good luck with in your Tahoes?
5. If I were to remove the tape deck, do they make a sort of replacement piece of fascia that could turn that spot into a little storage pocket or at least cover the area?

I was considering this head unit.

Thanks,
Erik