Ooh, thanks for starting this thread, Nyama - I'm about the same age and shape as you, and also have a party coming up.
There have been some great suggestions, so far.
I assumed I was pretty much limited to a plainish shirt dress (patterns clash with tattoos), as I have knobbly knees and veiny shins, so need to wear thick footless tights. I also dislike the 50s style (flat, saggy old butt).
A jumpsuit hadn't occurred to me, so I'm checking some out, now - thanks to whoever posted the Coast one.
Anyway, I would echo a PP, and have, in fact, made an appointment to have my long, boring hair chopped. I was googling 'edgy bob' even before I spotted this thread.
Is a radical new haircut an option for you, OP?
So far, I've bought a denim shirt dress and hope to dress it up with some amazing shoes - and the new hairdo. I've ordered a few pairs, so if I like them enough, I might post a (headless) photo!
I'm also watching a few Vivienne Westwood dresses on eBay. My friend, who is similarly scruffy, pulled off an 80s look for a recent wedding, in one (she's naturally more stylish than me, though).
It's quite a big deal for me, to ditch my 'uniform' of identical (Topshop Leigh) skinnies and (H&M) vest top. I'm very conscious of looking like a transvestite if I try to be 'feminine' or a NHS employee, if I get the shirt dress wrong 
What do you usually wear, OP?
(I will probably end up just going as myself, though, and dealing with the snide comments about not making an effort - my sisters are both younger, smaller, more able to afford clothes and shoes, etc, and my mum looks and dresses younger than me!)
Anyhow, I look forward to seeing what you go with, OP 