What are the most cost effective handling upgrades for the Octy vRS?

I am looking to upgrade the handling mainly to reduce the understeer that is prevalent when 'driving enthusiastically' and also to aid with getting the power down. However, I do not want to lower the car by much due to the speed humps that seem to litter many towns and cities.

My car is used as my daily drive so obviously I don't want to totally destroy the ride quality, but I understand that to improve handling the ride quality will be compromised to a degree. When I have sorted the handling out then I will sort out the power - but I want to ensure that the chassis is up to it first.

My choice is likely to come down to one (or a combination) of the list below:

• Upgraded rear anti-roll bar
  
• Upgraded front anti-roll bar
  
• Upgraded springs
  
• Upgraded dampers
  
• Coilovers
  

Which combination would give me the best performance for around

Save a bit more money and get a Quaife dif fitting. Gives you better traction off the lights, better cornering and doesn't effect the ride quality.

So as a serious question what can you do to a TDi since it's just the same base octy as the VRs that will tighten up turn in and make it handle properly:)