I wouldn't be touching a Z anywhere near the 5k mark! Its the bottom end of the market for a reason.
It really depends what you want the car for, if its for the engine and a bit of fun on the weekends an 03 JDM would do you proud but if your looking for a top spec car with all the goodies and a service history be prepared to pay 8k. Yes they are near enough the same age and the same engine but the spec/colour on a Z changes the price drastically.
According to the info on here, jdm cars "can" have all the same equipment as UK cars.
The only thing i have not seen mentioned is cruise control, it doesnt seem that popular on other jdm cars so dont know whether the jdm 350's have it, maybe someone could confirm?
Yes they can have but not very many do. The most common JDM's are no leather, no brembo's and usually auto's. You could be lucky and pick up a decent spec one at a bargain price but my comment was for the norm not the unusual.
Good luck with the hunt. If you stick a link up on here when you find one, we can all help you out!