My God!
Look at that list! 
Here's what we did with/ for my DD last summer.
I bought her a sweet, short dress from ASOS (£40 ish) which she has been able to wear again.
Ditto a pair of strappy high heels (£20) and a little clutch (£10) from New Look.
She needed her hair doing around that time anyway, so we made the appointment with our regular hairdresser for that afternoon so her hair was blow-dried nicely (it was just long and loose with waves)
We then went to our local department store where she had a ("free") make-over pre-booked with the Chanel lady who did her make-up beautifully. She then bought the lip-gloss that the lady used (with some birthday money - special treat).
As concerns beauty treatments - they day before, I did her a manicure and pedicure at home and we went halves on a professional spray tan (£18, so £9 each).
She wore some ear-rings that she already had, from Top Shop.
Lucky for us, it's deeply uncool at our school to arrive in any sort of show-y vehicle so she and her friends were just dropped off by their Mums and Dads.
Similarly, corsages are not the done thing here.
I'm happy with all of this, to be honest. She's a good girl, worked really, really hard for her GCSEs and was leaving to go to another school for sixth form so this prom was the end of an era for her. We have some gorgeous photos of her and her friends looking very beautiful and the whole thing is now a very lovely memory.
We'll do the same all over again next year for my younger daughter - by which time the older one will be having the older one's Sixth Form Leavers Ball.
... and on it goes!
Re: The dreaded "same dress" anxiety: My daughter and her friends all took a photo of "their" dress (just hanging up) absolutely as soon as they had bought it - and posted it (or a link to a picture on the shop's website) on Facebook saying "My prom dress - please don't anyone else buy it!" or whatever. This clearly "baggsy-ed" it as you can see who bought what first, because it's all documented by date.