High idle is more than likely a vacuum leak, unless it's a tune problem, but you won't know that until you get the other issues squared away.
Trace back vacuum lines, and tie up wires/etc, simply clean up the bay. This gives you time to check for leaks as well.
I'd suggest you swap plugs/filters/oil in the meantime, turbo cars require a fair amount more maintenance than your typical daily driver.
The mainfold looks like it was mig welded, and is just old and suffering from heavy rust.
I'd also take the time to do a compression check, get a good base line of the motor, so you can tell if you have any future issues with blowby/etc.
05 Berlina Black S2000 700whp / 500wtq