Congrats and welcome!
> I want to add a center gauge console, Is it already prewired for one ?
I don't know for sure about the wiring on a 72. If I recall correctly, the center console with gauges was a 73+ option.
> I bought some nice Fuchs replicas. What size/brand tire do you
> reccommend ?
I hate to burst your bubble, but I'm not sure there is a such thing as a nice Fuchs replica. They're heavy and prone to cracking. That said, tire-size is dependent on the backspacing of the wheel and how much bier Hans had at lunch before he fit your fenders.
Generally, a 205 will fit under 914 fenders. I have 195s on real Fuchs on my 73, and I have plenty of room left.
> Can I use the lug bolts from the steel wheels ?
Not advised. Alloy lug bolts are longer than the steel lug bolts for a reason. Use a bit of anti-sieze and torque them to tighten. If you gorilla them tight, you risk cracking a wheel.
> Weld in or Rivet in a new battery tray ?
I've seen both done. Riveting always seems a bit mickey-mouse to me, and might cause you to fail tech-inspection, should you ever choose to autocross the car.
Welcome to the madness!
-Rusty