I've had a couple of MGFs.
When they've been standing out in the winter, I've put a plastic groundsheet (like for a small tent) over the canvas roof, and then a cheap car cover (it was about £20) to hold it in place.
That seemed to keep the soft roof in pretty good condition.
Also, when the car was cleaned, spray the roof with (1) soft roof cleaner, and (2) the reproofing stuff. They have it in Halfords, in white bottles with coloured square labels on the front. Some company who do loads of car cleaning stuff.
But, go for it. Once you've had a soft top, you will never again want to be without one. They are soooooo much fun.