NEED ADVICE : WHAT TECHNOLOGY TO USE?

- WHAT TECHNOLOGY TO USE? I need recommendations, on how to implement a web based program for car rentals who share data about their cars. I need to know what framework is best: 1) what application layer technology(programming language) to use and why, 2) and same with web-layer, what technology to use and why. Requirements: a) the system will be fast(scalable). b) The web technology must have the potential to be cool, powerful(contain flexible and cool looking tree control) The business logic is quite simple, there will be access control, car-rentals can invite other car-rental to view information on their cars. There will be a tree control and a list of cars on some pages, search criteria for cars(model, color, engine size, etc) I want this web system to be implemented in a way that I would not need to reconstruct the system from scratch if all car-rentals in the world were using it. Imagine that the system is for thousands of carrentals, each having hundreds of cars. Fast and scalable are the keywords. I´m looking for "to the point" advice on the questions numbered 1 and 2 above. No time wasters please, as I have already wasted quite some time on other sites.