I have all my cars windows tinted. I find really dark tints look terrible/cheap and are dangerous during dark o'clock. I normally go for 35% film. I've found 50% and above and there is no visible difference to standard. Driving in strong sunlight gives me a sore head but a slight tint makes the whole cabin more comfortable.
Remember, as a general rule if you have a dark interior the tint will appear darker, with a light interior a weak tint may not make difference to appearance of car. As for aesthetic value, it's all down to personal taste. I think done properly and at the right level it can transform a motor.
Around £180 would be a good price for a Z. Worth paying for quality, there are lots of wannabe amateurs out there.