Wish I could have afforded a 4/5 year old car when I was an apprentice, let alone before I started.
Just jealous mate, nothing personal
Given that it has the highest power 1.5 under the bonnet anyway, not sure what would boost that or even if sensible to do so.
One of our guys is looking at tuning boxes on an 80hp version so perhaps keeping an eye on that one.
Styling, given that you want to revert back for selling, I'd avoid anything that isn't simply bolt on.
Wheels, Tyres and Suspension probably but IMO, drive it around a bit first, see what annoys and what is good.
Base upgrades on that.
At your age, I was happy with a decent sound system, an engine that started and a passenger seat that reclined easily