Question about auto gas STC

I am currently interested in an older C-172 (Cont. 0-300 engine) that has an
STC for auto gas. My questions a

1. Can I continue to use AVgas instead of auto fuel?

2. I often fly into/out of Big Bear City, Ca. (elev. 6500ft.) I've been able
to get out lightly loaded (me and one in the right seat) at a density altitude
of 8,100 with one of our clubs 172s with the 320H2AD engines at 160 hp. What
difference would I see with the 15 fewer horses up front?

3. Can you give me a rough idea of cost to add a 2nd Nav-comm with Glideslope
and then have the whole thing certified for IFR? I would probably try to find a
good quality used one from one of the local shops.

Thanks for any help you can give to a rookie getting ready to make a first
purchase.

Jim