A T30 could have the larger 340mm brakes as standard as the van's load capacity has nothing to do with the brake size.
That being said, a 17" steel is a common spare size and would fit over any VW OEM brake caliper. As would a variety of 18" steels.
I don't believe that 255/45/17 is a very common size but as an example, 235/65/17 is and would be very close to what you currently have. Certainly close enough for a 2wd van, if that's what you have.
215/65/18 is a perfect match but availability and load ratings would need to be checked.
Have a play on "willtheyfit.com" to compare sizes.
However both your current size and any equivalent is a lot larger than the 265/40/20 mentioned above. About 20mm in diameter in fact.
That will be difficult to fit in the spare wheel carrier without modification.
