no need to upgrade internals, they should be good for 300bhp.
You could see 240bhp on the standard k03 turbo but best bet would be to swap this for a k03s (around £300 exchange)
You'll then be looking at replaceing the standard weak and restrictive parts, starting at the intake you'll want :
a replacement TIP (forge etc)
007p re-circ valve (to hold higher boost you'll be running)
either a modded airbox or replacement air filter (BMC, Green etc)
Then onto an intercooler, Forge sell a replacement FMIC which is proven but there are others around from universal ones to Renauly laguna items (search the forum)
onto the exhaust, you'll want to replace the downpipe and CAT with a free flowing item such as blueflame or Miltek and the corresponding full system to go with it.
Once all this lot is bolted on you'll want to get this remapped, REVO, APR, AMD, CC, take your pick, you may need a 4 bar FPR depending on the map.
If you take a look at Dan Payne's Cupra-r for example, he has all the above mods and CC map and makes bang on 240bhp which in my opinion is prefect power for a fast road cupra. Nice and drivable, reliable and still usable daily.
Price wise, check the for sale section and ebay for bargains but you'll be looking at around 2k for all of the above.
From there you could add replacement manifold which should
help the turbo spool quicker, JBS do a good one.
Don't forget the handling and brake department though, brembo 4 pots are a good upgrade up front, as are 15-20mm rear spacers for stability. Lots of different suspension packages from struts and springs to full replacement coilovers.