Hi, well done for getting short listed!
I have worked for next for 5 months now, and Ive only just turned 18, so your age wont stop you with alot :P
The salary changes alot, as you have to do quizzes throughout your training and this willcause your pay to increase. The clothes allowance is worked out on your contract, and as heard, is usually 75% off £75 sounds alot but is really only enough to get you a dress and a top (I wear my own black trousers). However, its all you really need for a part time job
The job itself I really do enjoy. I hear alot of people complain about working for next, but I havent had any major problems. The interview is straight forward and the lady who interviewed me, who is now my manager is so lovely. The other staff are really nice too, and its just all about having confidence. You are often put on different departments, to get a feel for the whole store, but having diversity is fun, and if you're not sure of anything you can always ask somebody. It takes a while to get used to using the tills, and doing directory returns, but becomes pretty straight forward soon after. They sometimes may put you on a shift that isnt your contract, which is great f you want more money/experience, but if you cant do it, you can speak to the manager, and possibly offer some other time. Ive done this, and although they may rely on me just a tad, they appreciate it all the same.
Good luck with everything, let me know if you get an interview or the job!