I recently had a set of wheels powder coated for my everyday car. In the past I have used companies as far afield as Birmingham for the service, but this time opted to pay a little more for local service and chose Willow Powder Coating in South London.
Their service was very good, they can blast and powder coat a wheel within a week and normal surface rust doesn't seem to be a problem. For my everyday car I chose Gloss Ford Diamond White for a rally look; but Silk finish silver, Silk/Matt black or many other colours shouldn't pose a problem.
Expect to pay around £50 per wheel and remember to deliver them stripped of tyres, balancing weights and valves, etc.

Powder coating - Before & After

If you want to do a classic Opel or Vauxhall Rostyle wheel, you may get best value by choosing a Matt Black finish and spraying the silver sections yourself. Powder coat takes paint well providing you lightly key the surface by sanding it with 600 or 1000 grade wet'n'dry sandpaper. If you don't have a paint factors nearby, you can buy wet'n'dry sandpaper at a reasonable price from Screwfix, they also stock production paper.

In the past I have used Hammerite Silver Smoothrite paint to excellent effect, I have also had good results from the range of industrial paints available by through CPC Farnell, such as their excellent "Steel Wheels" paint from CarPlan. The last set of Opel Rostyle wheels I had powder coated was around a decade ago, they still look good today and that is with storage outside in all elements.

If you want the best finish for your original Opel or Vauxhall wheels I cannot recommend the benefits of powder coating highly enough, which outshines and outlasts the original finish.