I use an Xcel Revolt X2 5/4mm (
https://store.magicseaweed.com/Xcel-Wet ... uct/231023 for reference) for surfing in Scotland and I cook! It's almost too warm in the winter with boots gloves and hood on and that's at around 10 degrees C water temp and air temp of around 3 or 4. It's snowing here this weekend and I was out yesterday, had to take my hood and gloves off for a bit as I was starting to melt.
Thicker you go the more resistance for paddling too. Sure you need 6mm? Only guys I ever see with 6mm suits is this time of year up in Thurso which is as north as Scotland can go and can get down to 5 or 6 degree water temps with air temp being -5 or 6 on a cold day. It snows while you are surfing it's that cold.
Better quality and thinner is more expensive but if the suit is going to be a long termer it's worth it. Can't rate the Revolt X2 highly enough, it's stood up to the entire winter here and not once have I ever thought it not warm enough. I can be out in the water for 3 hours and come in for a break because I'm tired instead of cold.
Boots/Hood/Gloves, better to be thicker. You lose most of your heat through your extremities like top of the head or feet so good boots and hood really help. I went for the Billabong furnace 7mm internal split toe boots and rate them. Properly toasty, excellent fit, no sliding around in them and pretty hard wearing so far. Hood wise, mine is rubbish. 3mm but it's got holes in the ears to let you hear which totally defeats the purpose as it lets water flow in. If you are duck diving, your hood will be full of water by the time you get out back. So my tip, go for one with no holes round the ears!
Gloves wise, I have a pair of 5mm things I got on offer as a last minute "damn my hands are frozen" purchase. No idea what brand, they are warm enough but they are the mitt type which are a PITA. I'd recommend the lobster claw ones (I don't think you really get single finger gloves above 3 or 4mm) as they are probably a bit better for using your hands than mitts.