Once you have a watched a few tri's you will realize that you will see all sorts. I have seen breast stroke and backstroke used, I have even see people with such poor freestyle that they are verging on none swimmers. Look at previous results and I would be amazed if there wasn't a few old timers who are probably on there 100th tri but have times far slower than you are aiming for. If you had no concerns then you would just have a smaller sense of achievement when you finish.
Don't worry about tri suits, I raced for years in swimming trunks and faffed around wasting time in transitions with running vests. Specialist gear can come later. Again I have seen numerous people toweling, talcing, putting socks on etc. no one cares and you'll probably make someone else happy when they see someone else just enjoy event.
Normally you will be given a start time relative to your swim pace. There are normally lots of breaststroke swimmers, so no need to worry at all
The swim is relatively short in a sprint, even if you swim slowly, you will only loose a few minutes, so not going to be a big deal
Don't worry and enjoy yourself
Breaststroke will be absolutely fine, and as others have said, you will be 'seeded' according to your estimated time so you'll be with swimmers of similar ability. You will indeed see a wide variety of strokes and abilities, some of which will be amusing, others curious and still more like watching someone drown with a modicum of style. The bigger thing is that you and all the others are putting yourselves out there and that is far more important than how quickly you complete the leg.
As for a tri suit, it's up to you. You could have a change of clothes for each leg or take away the faff and stress and buy the all-in-one. It's up to you, your goals and how you feel about changing kit in the middle of a car park(!). Given that you've only got two weeks now, though, you'll need to buy a tri suit soon if you already haven't and do some swimming, cycling and running in it so you get used to the feel and sensations.
I also face this problem, but the pieces of advice you give is so helpful. Thank you so much.