Autotorque is a good bet. I personally wouldn't use garage D on a GT-R. They are skyline specialists but there's no similarity between the models that makes their experience any more valuable than a normal mechanic.
Personally I'd make the trip to Litchfield. I work in Bushey so have made that exact journey on a number of occasions.
Reasons I suggest this is because the drive itself is fun, the facilities there were good before but soon will be exceptional, and also it's like going to a cool toy shop. Only downside is it's difficult to avoid buying toys.