Go big or go home
Look for boat ranges with an "HV" (high volume) or large option.
The most important factor to you is going to be how well the boat holds your weight and how well it handles under that load. Most boats have a design weight for the paddler usually capping out at around 130kg (285lb) mark for high volume boats. Allowing for a reasonable amount of cargo weight for a touring or sea kayak that should be enough for you.
Length is speed
If you're worried about keeping up with others, go long. There's a general (to a limit) correlation between boat length and hull speed. The longer your boat the faster you'll be able to go in it.
Seats can be replaced
Don't worry too much about outfitting in boats, it's reasonable, and entirely normal among some groups, to completely strip out all the fittings and replace them with something more suitable for the paddler. Boats often come with "club" outfitting, which is the most basic and hard wearing version with options to upgrade for more comfort, better fitting, better padding etc. Speak to dealers to find out what the options are for the boats you're considering.