Assuming you are an adult, and not just a high school dropout stuck at home, there is nothing wrong with starting on soprano. I will say that if you plan to play in public, with other people, at some point, alto or tenor will give you many more opportunities. But for just making beautiful music, there are lots of people who play soprano only.
£300 is a short budget for a good quality instrument. Sopranos are at least as much if not more expensive than altos. £500 would ensure that you can get a good quality instrument. Yamaha sopranos are more than the “student” level altos, if only because their soprano line begins at the “intermediate” level.
Regarding the shop setting up your horn, shining up is probably more like it, especially for a used horn. Some one blew it for 5 seconds, it made a noise, they wiped it down and said “good to go”.
I don’t want to be the one who rains on your parade, so take what I say as warnings and a recommendation to keep your eyes open. Welcome to the forum!