I think you'd have to be keen to have a wideband set up to justify the change. Also probably worth fitting the VVT tensioner at the same time.

The cost could vary a bit depending on how you go about it. One donor vehicle would be the ideal solution. Golf/A3/TT being best in my opinion. Though you could do Polo parts but the fitment wouldn't be so hot as the ECU is bulkhead mounted.

Gavin