From one Big guy to another (well, not anymore), I've been in business for over 20 years. I've gone through a handfull of mowers, and I can give you some advice, about seats and room for your legs.... I have tested just about every mower on the market, right down to the china made ones, and only having a John Deere for 2 weeks...... I am much happier with a Deere, than all the others I've owned or tested.. I work by myself now, but if you are going to have helpers, you should be spending more time on the mower.
I was up to 364lbs at 5'8" and I had the knick name "Fat-Boy" and "Porkchop" from a few of my mowing buddies... The Exmark that I've had, beat the devil from me on bumpy turf, and after 10 or so lawns, my back was killing me. I would have rather done anything else other than run the mower... give me a trimmer! Other things that I disliked about some mowers was the room from the sticks for my legs. I always seemed to not be comfortable and be able to relax my legs, I needed to hold them together. Now, after loosing 140lbs, I still found that some mowers just didn't seem to "fit" me.
What I found about using the John Deere 900 series is that there is a LOT of room and a comfortable seat. I mowed 20 lawns the second day, and did not feel beat-up. Even after sitting for 5hrs mowing a complex, I still felt relaxed. Another thing that I liked about the JD over my previous mowers was that I didn't get covered in dirt from the kickout from under the deck.(Mulching) I could go on about other likes and dislikes about each mower I've owned, but so far, I am very pleased with the z910a with a 54", but I would consider going with a z920a or larger if I was over 300lbs with a 54" deck.
The seat on the Deere is thick and seems to be wide witht he arm rests being narrower. My last mower, I had to remove the arm rests because my backside wouldn't fit between them comfortably...
one more thing, make sure you go with a wider tire, it will make a difference over time.. being heafty, a wider tire wouldn't hurt. Good luck