I'd go for simple upgrades first, the simpler the better and ease yourself in.

The brakes will take a hammering, so stick some uprated pads in, maybe some uprated OEM fitments discs or at least make sure what's in there are in good condition.

Also make sure you have four matching tyres and that they're all in good shape. If they're not or they're ditchfinders, change them for something decent.

Make sure your tracking is sorted.

Then take it on track then you should be able to make your own mind up on what can be improved. A set of Eibach springs are a good, cheap mod, but if you're serious and want to go that far then coilovers are always good if set up right. Then there's ARB's, polybushes etc etc etc, all depends on how far you want to go.

And you can go as far as you like with brakes, no shortage of big brake kit options, but ST170 brakes would be a budget upgrade with uprated pads etc or keep your eye on eBay for Focud RS Brembos or other cheap OEM Brembos getting sold. Or K-Sport do good, but cheaper BBK's.

Then look at lightweight wheels to reduce unsprung weight.

After that it gets serious, but that's up to you whether you want to go down that road or not. You'd have a very decent track car by then.

But from day one strip out as much weight as you can. That's free and easy and shedding weight is equal to gaining power.