I know how you feel, I was exactly the same when I was buying my first shortie.
My advice would be to go with a wide shortboard to start with, mine's a 19 3/4 wide. I came down off of a 7'9'' Minimal, and was advised to keep some of the width, so it wouldn't be such a big shock and disaster!
Mine's 2 1/4 thick, which makes it ideal for learning to duck-dive on, I had it mastered within the first 3 times, unless you can do it already that is!? *falls at the floor to your feet* if you can! My minimal was a bugger to push the nose underwater, let alone all of it!
I got a new custom one made for me, which I dont know wether it was wise (?!) but I had had experience of shortboarding before; and im getting on really well with the custom. I cant really say get a second hand, cause that would make me a bit of a hypocrite?! But what I can tell you is I got on fine with it.
One thing I wouldnt recommend, is that AlMerrick. Thats something you must definitely not do: get a performance board as your first shortie. It'l be a shock-to-the-system and you'l be frustrated your mates are getting waves when you arent!
Anyways, the best thing to do is to get your ass down to a proper surf shop, maybe a shapers (?) and get their expert opinion.
Hope my amateur opinion helped slightly!
What breaks do you surf in Wales by the way? I remember going there as a kid and watching these massive sets roll in from a relatives rooftop! (that was before I surfed!)