a look around a local shop. You must try this stuff on as fit is crucial to ensure the armour is in the right place and will stay in the right place during an off. Work out your budget and have a gander at what there is.
If you're getting two piece make sure they have a solid zip that goes all the way around.
As for brands, there are a load out there and they'll all have their bottom end cheapo ones and their top end ones. Go out, try a load on, see what fits and then look online for reviews.
There is no point spending £1000 on amazing leathers that don't fit properly as they will not protect you properly in a crash. The armour will be out of position, the seams may be put under undue stress (as the stress is happening in a different position to where it was designed for) and could burst open.