Did they actually comment on the clothes you were wearing at the interview? If they did I find that quite disgraceful and totally in appropriate.
I am sure with your experience you were dressed in a manner fitting to an interview , where your outfit came from is totally irrelevent in my view.
If I were in your position now with an offer of a job on the table as it were , I would take it . I would wear the best clothes I could reasonably afford , smart serviceable and appropriate to the job and then I would make damn sure that I was the best rep they had ever had by being excellent at my job.
Would suggest that you aim for 2/3 good quality outfits eventually, which you can dress up as you wish and accessorise(sp)and even interchange In the job you will be able to network and make many contacts, at the same time keep your eyes peeled for a better position wnen you have established yourself , and it will happen.
Don't worry a scrap about designer stuff , people who focus on things like that totally ,are only deceiving themselves that they look good ,when in fact they are being sucked in to one of the biggest commercial rip offs of all time.The verybest of luck , please if you can let us know how you get on Go for it..