With regards to Mods well it can be small and discreet or it can be in your face and way over the top its all down to how you want your motor to look.
Go through peoples readers rides and you will see what others have done but don't forget if you modify the car you need to let your insurance company know.
With regards to more power again the sky is the limit (or more about your bank account) but again you can see all the stuff you can do to engine / gearbox / suspension so get reading.
Finally check out
OBD11 / VCDS tools (there are others) these allow you to modify how the car works for example Scandi Lights means your have rear lights working as rear DRL'S.
Things to look out for:
Water pumps are a weak link so check to make sure its been replaced, if not you may have to pay for it.
You state that its got a full
service history check what that actually means
check out this
list and make sure all these things have been done
Heater Matrix are know to get blocked (lots of post on this subject) so make sure they have the correct header tank (one that does not have a bag in it)
I would ask the seller if any
OBD changes have been made (eg have the configured the car to be slightly different to factory)