I own one of these, a 2009 model and have learnt quite a few things in the last few months.
They are a good car and very very tunable.
Depending on mileage, first thing I would do is put on some decent tyres, and have the money ready for a new dual mass flywheel and clutch when you need it. Loads of people give a car more power and start to get clutch slip or flywheels failing.
First upgrade is going to be a stage one remap which will take the 1.8 to between 190 and 230bhp depending on remap, you will also see a huge jump in torque, and it's quite a torquey engine anyway for a 1.8 petrol.
If your talking upgrading the turbo then there's turbo kits out there for a few grand which will take you to over 300bhp, but your going to need to upgrade the exhaust system, probably the injectors too.
Depends how far you want to go. I will be keeping mine stock, if anything I might put on a miktek exhaust system, more for sound and cosmetics than performance. Maybe a stage 1 remap after my clutch goes.
Insurance costs are my biggest concern.
Message me if u have any questions.