Dampers
Alignment (including hardware)
Wheels & tires
Brakes
^ All at once if possible. It's a broader concept of 'suspension' but IMHO you need to consider how everything on the chassis works together when it comes to effective track setup.
If you don't have somebody to set up your dampers for you I'd go with the single adjustable Ohlins given your budget. Not much point in multi adjustable coil overs if you don't know how to set them up. If you have somebody to set the dampers up for you then Bob's your uncle. KWs, Teins, JRZs, whatever. .
I mention tires because there's a pretty big difference between the OEM rubber, extreme performance rubber and the semis licks/slicks that some folks like to run.
Brake upgrades for safety and consistency, plus if you're going to go with a BBK it might influence your wheel & rubber choices.
Alignment is pretty easy to figure out once everything else is dialed in.
Generally speaking, quality parts and fewer variables = less head scratching and more time spent focusing on driving technique.
IMO.
