I'm no skinny person at all, and had no problems whatsoever with the 150 today, funny thing was a few weeks ago someone told me I should get a 275, I went to the Chandlery and they told me the 150 would be fine, today proved it, even fully kitted up with wet weather gear.
How they can charge £19 or so for the crappy spray hood is beyond me, but it's not so crappy when it's saving you from drowning
Check the cheap and nasty plastic buckles and belts on some of the lifejackets, in a real life situation these will be tugged and pulled, the cheapo ones will snap, I saw some today, so get the best you can afford, when the time comes and you have to use it you will be ever so grateful you got what you did.
I guess if you are on a budget you could make up your own crutch straps but DO have one, as I said it's as good as a chocolate teapot without it on, I have not been wearing mine until today, I learnt the biggest lesson when I leapt in to test it without one on, scary indeed trust me. I fitted mine this evening.
I also learnt so much about liferafts today, but I guess that is for another thread, you WILL NEVER know what you need out of one until you do the training course, and have to right the liferaft with people inside, even getting in in the first place with crap rope ladders, it's a killer in a pool, never mind a freezing sea with waves and wind. If you have a life raft DO the course, without doing the course you will be in for a massive surprise and possibly die if you have to use it.