what does your tyre sticker say inside the door?
Mine says something like 100/102c.
dont forget its not just the tyre rating either, I have a lwb caravelle and had some 18" RS8 reps which had cast in the rim 690kg which is the maximum load of the alloy, with the `velle the wheels have to be rated 800kg+ Its when you dont use correctly rate wheels and tyres is when you get buckled/cracked rims/blow outs. People do run alsorts, but its if Mr Plod pulls you he might get a bit funny too.
If you can get 103s (new transit I suspect) they`ll be plenty. Or as mentioned look what new transits run, pretty sure you can get 17" alloys on them.
Its like being chased by an imploding star...
...One day I'll finish it all.