Welcome!

I was similar to you about a year ago, I didn't know too much about the mechanical side of cars but I knew I wanted to start with a 1200 since I'd always loved them.

I also bought a very stock 1200, mine being a coupe, and made a list of the things I wanted to do based on feedback from the guys on this forum. Here are the things I started with:

- Larger diameter sway bar
- Electronic ignition
- New springs
- 32/36 Weber install (in progress)

Once I got some confidence, I moved on to some more difficult upgrades:

- Coilovers (in progress)
- Brake upgrade (in progress) - I personally went with the 280zx upgrade for a few reasons, although they are very heavy. The more popular choice would be the s13 upgrade

The widest tyre you can go with largely depends on what wheels you can find to fit given the different offsets, but it seems like 7" is as big as you can go on the front without body mods. Although finding a wheel/tyre combo to tuck could be difficult. On the rear you can apparently fit up to 9" wide. More info found here.

Air conditioning is possible, it was a dealer option. Units pop up here ever so often.

As for what to do with the engine, I'm sure one of the other guys could give you a better answer to that than me, however, popular choices are covered in this section of the wiki. The current favorite seems to be the CA swap.