5 speed manual box (don't know what options exist)
280ZX / S2 bluebird '71B' is a bolt in.
You can fit a 71C box from an S13 or R31/2 Skyline, but it requires mods and is not DIY friendly.
You will also need to consider your diff ratio, as the auto diff is very tall.
Subaru A/C system
I've not heard of this. Another option is an AC system from a later 260Z is a bolt in, or you could go Vintage Air.
Possibly a body kit (flares, side skirts, air dam, haven't seen a spoiler I like yet.)
Each to their own, but none of the bodykits of seen for a Z improve the looks.
Possible after market front suspension to eliminate rumor of bump steer.
It's not a rumor and it gets worse the more you lower a Z.
And it's a significant undertaking to deal with it, and none of the adjustable suspension kits on the market adequately deals with the issue.
I suggest you speak to a Zed specialist - either Les Collins or Stewart Wilkins.