For me, trackdays are all about fun. So I'd go down the power and brakes route first. Though for what I would like to fit, it would quickly swallow up £2K.
Even if just a remap, your looking at £300 at the cheapest. If you're lucky, you're already running brembos so Performance Friction brakes will 'only' be about £1000 including decent pads fr front and rear.
That takes care of go and stop. Handling I'd start with front and rear ARB's, ALK and a full geo setup.
That'll leave you with spare cash to carry out to a service after the big day and some towards the imminent need for new tyres.
Of course savings can be made on the above buy either buying alternatives or going down the second hand route.