I had an old Beetle that used 4 lug bolts and I used adapters to mount Pontiac 5 lug X 4.75" wheels on it. I drove that car through creeks, over rocks, and at speeds way past legal. I never had any problems with the adapter bolts or the lug nuts coming loose. The ones I have are cast aluminum. I would expect billet ones to be even stronger, but like I said, I never had any problems with the cast ones. I drove that car into the ground with those adapters bolted on it. I recently bead blasted them and checked them for cracks. I found none and they look good enough to use on my next VW project.
If you decide to use adapters, you should get your front end aligned with the new wheels and tires and adapters all mounted how it will be driven.
Here is a link to a good looking billet adapter. http://www.adaptitusa.com/index.asp?...ROD&ProdID=171
If you order some online, make it very clear which bolt pattern is which, they make both kinds. That is to say, you wouldn't want to order Cavalier hub adapters for Bonneville wheels, when what you need are Bonneville hub adapters for Cavalier wheels.
These are $45 or so apiece. Is the $80 price you were quoted for all 4 or just one?
$20 apiece sounds kind of cheap. If it were me, I would get the best ones I could find. These Bonnevilles are a lot heavier than a VW Beetle.