What size do you mean by smaller?

What size are the original wheels?

To small a wheel would look sully but one size down wouldn't look to bad.

Don't know about better resale - I'm not familiar with the car. However, regarding petrol v diesel on your mileage : I do similar and would never again by a diesel. Although the car itself is great I have been stung for a new DPF ( around £1,000) and unless you do suitable miles petrol, in my view, is a far better option.

Go for the 18" wheels - the ride will be much too hard on the larger ones. Young men overstocked with testosterone tend to go for larger wheels because they think they look macho.

I agree that at 9000 miles a year you should go for petrol. At the moment diesels hold their value slightly better, but there are many of us who think that with all the things that can go expensively wrong with high-mileage diesel engines, this differential will disappear in a few years.,