For starting out I'm in favor of the full footpocket monofins. I have a Special fin Freediver model that is very comfortable and I can wear for hours. The blade has an angle to it so is comfortable for my stiff ankles. If you have flexible ankles then you can go with a straighter blade- like the Waterway NEMO fin.
I had a Dolphin carbon fin like Sam uses, but it tore up my feet- even with socks on! The fin itself wasn't as expensive as a full foot model and more closely resembles true fin-swimming fin, but for starting out comfort is key. Here's what my feet looked like after two pool session with my Dolphin mono.
I also included pics of the dolphin fin, freediver mono, and NEMO fins that I have owned. The NEMO fin has the most comfortable footpockets of all, but no blade angle. Whichever you get I would go with a soft(ish) blade to start with.
For your temps I would think a 5mm would be more than enough- but then I have more natural insulation that fondueset does.
Jon
