Wow, that’s a tough one. I’m not sure what’s available these days at all.
The real answer will probably vary depending on how much work you want to do.
I like the Bachmann RailTruck for sheer flexibility. I modified it to be completely different, but it was a lot of work.

But, I don’t know where you’d get one. (https://www.largescalecentral.com/externals/tinymce/plugins/emoticons/img/smiley-cry.gif)
Something “easier” might be to use the H-L-W Mighty Mack.

I don’t know if you can just buy the block, but Robbie has them in stock.
I use the Revolution for my controller - sometimes with a Phoenix sound card; sometimes with just the Revolution sound.
You MIGHT be able to find an Aristocraft or USA Trains block, but I wouldn’t know where. I think your main problem will be availability.
Another really DIY idea is to build it up yourself: http://www.sdp-si.com/ although I seem to remember somebody offering a ready to go product - unfortunately my memory seems to have gone elsewhere…
What’s the VW body like? Would a motor block slip in? Or, are you trying to make it look like a VW with 4 RR wheels?
More ideas (with no idea how practical they might be, or even if they are affordable):
https://www.precisionmicrodrives.com/gearmotors
https://www.modelmotorcars.com/scalehardware/
http://hobbymasters.com/science-kits/motors-gears-wheels-pulleys/
Sounds like a fun project!