As I own a PA-14 since 2013, I think the kayaks 600 pound capacity is just what you need....and that you have identified the seat weight limit of 350 pounds.
I really don't know how much fudge factor over the 350 pound limit Hobie holds close to their chest, no doubt it can handle a little more weight, but the seat is expensive to replace if you break it.
Losing weight, to get you under the max weight limit is smart....once you are ready to go, using the PA-14 will give you a good workout and help to keep you fit and maybe even lose more weight!
Good luck with your dieting