JV and I are doing this next year.
The good thing here is that, if you're building this just for track days (rather than competing in a race series) you can get on the track RIGHT NOW!!! and carry out your mods over time if you want.
First things first is a helmet - I've never been on a track day where the let you on a track without one, so invest there first.
A fire extinguisher to keep in the car is a wise purchase, just make sure that it comes with brackets that they are man enough to keep it in place in the event of an 'incident'
As for the rest, the order in which you do it is up to you. Suspension will allow you to maximise your speed with your current engine (by increasing cornering speeds) but won't increase the speed at which you arrive at the corners.
Zorst and filter will give you more grunt on the straights, but ifyou're still running stock suspension, you aren't going to run any quicker through the twisty bits.
Seats/harnesses are nice, and will make the day less of a strain on your back, and should you have a shunt will protect you further.
A cage will offer improved safety in the event of a big off, but you're likely to get flagged by a marshall if you're driving hard enough to provoke a roll, long before it happens!!!
The best people for all this stuff under one roof is Demon Tweaks, as they are quite focussed towards track day gear/entry to motorsport etc. Give them a call and get a catalogue.
See you on the track next year buddy