It needs to be full length. I am 35, 5ft 6/7 and a size 14-16. Usually a 16 as I am a 36H.
I quite like this, but feel like it's a bit boring and I have no clue how I'd be able to wear a bra with it anyway. 
www.coast-stores.com/tobey-crepe-maxi-dress/dresses/coast/fcp-product/2213156589
I could maybe do strapless (although would probably wear a shrug type thing or wrap) as those Gossard ultimate strapless bras work well on me. I am happy to flash a bit of clevage, but still want to look classy. £350 would be about my maximum budget, preferably under £200.