not that common, I had 2 mk3s myself, lots of mates had some. I never had to redo the entire vac system; only once a broken bit of tubing on driver's door sill.
and when you've replaced vac lines they will last for another 15 years

I know some people who have retrofitted motors... they normally don't last nearly as long as the vac system.
I fitted the hawk remote locking kit to my 96 estate, was easy but the whole thing with also swapping motors is a different ball game and involves ripping half the car apart. you can do it, get yourself wiring diagram from haynes... or post the hawk installation instruction here and I'll have a look at my haynes and tell you how to connect if you really want.