Dont know what the bit of snapped plastic might be without seeing it, but if it's part of the tensioner then i wouldn't be driving the car until it's fixed. If the tensioner fails the belt will go flying and you'll have valves crashing into pistons.
£250 sounds about right because vw designed the mk4 engine bay in such a way that you have to remove half of it to get to the belt/tensioner.
Make sure he puts a new belt on too, even if yours is only a few thousand miles old.. for what it costs you might aswell give yourself another 60,000 miles worry free driving. If your car has done around 60,000 anyway, get him to put a new waterpump on at the same time, the part itself is about £25 and he should fit it for free.. since he has direct access to it with the belt off.
As for the famous MAF fault (air mass meter, screwed into the side of your airfilter box with a plug into it) , if yours dies, as most do, it'll cost you £75 exchange from the dealer.