I've found this website very helpful: http://www.lifejackets.co.uk/ as I have just decided to rearm all nine of my lifejackets. It includes helpful videos on how to do the maintenance.
I would only buy second hand if you can inspect first, and leave them fully inflated (you can do that by puff-power, and should do so every year anyway) for 48 hours to see if they leak.
Especially when wearing other winter clothing, I find you really don't notice the self-inflating ones - as you say the bulky ones get in the way, impede all round visibility etc.
You then have a choice between
manual inflation - not good if you knock yourself out on the way in. (I have 1)
hydrostatic (often described as "Hammar", a common make) which detects when you are a foot or two under water (I have 1 of these)
automatic which just detect water. In theory these can go off in very heavy rain or spray or if you forget to take it off when having a shower. This has not happened to me yet, (I have 7 of these!).
As to make, I have a mix of Crewsaver and Plastimo. Look for the EU number.
PS prompted by the question about the duck ring above: some (a minority?) have crotch straps as well.