Often the simplest least expensive dress is the one you value the most. I bought a black long dress in my early 20s from Miss Selfridge and it lasted 15 years with a once or three times a year wear during the festive season or an unexpected formal do. It was plain as plain. Very simple just like today's pick from H&M. When it gave up the ghost or rather I knew its shelf live had passed I probably sobbed!
I don't think you can go wrong with sticking with simple, almost obvious (a black shift dress that is) when it cost £24.99. H&M are having a moment, clear collections, my favourite being the conscious collection, well organised shops, some attempt at better customer service but really not so important as good stock levels, which always makes life easier.
You won't win prizes for originality but you will feel smug and if you use the going out shoes and accessories you already have then you deserve a making do badge!