So, as with any mobility aid, the correct time to start using one is when you feel like you need one. I've met 500 pounders with no mobility issues and had friends whose weight became an issue at like 250 because of bad knees. So if it's an issue for you use one.

If you're too embarrassed to ask your doctor about it you can buy one on Amazon, probably in the $30-40 range (depends on what you want style wise)

Not really pre-existing conditions, but still gaining despite developing medical conditions. Honestly the hardest to work around is a gluten intolerance. Though sometimes you've just got to spite your stomach for good food.
More seriously I've got low level diabetes, high blood pressure, and high cholesterol. If I watch my intakes of sugar and salt I'm generally fairly good. My doctor is aware I'm not planning on dieting so he's got me on meds to handle most the heavy lifting there.

if you want a 350 limit scale you just about have to go digital, most dials cut off at 300 or 330.

Personally i wouldn't bother with a dial scale. Once you can't see your toes easily the dial becomes a liability. With digital you can step back and see the weight for a few seconds

Walmart's website has a bunch that go to 400 in the sub $25 range if you want somewhere to start.

Definitely over the belly.
Jeans are real pants reserved for work or making a good impression.

I'm not bothering with real pants for anyone I'm comfortable enough with to wear them under my belly.

The sizes are always so weird. A C on someone with a small frame looks sizeable, but on a large frame it isn't as impressive.
By cup size I'm an F, but I don't feel I look that big.

Probably, but a ring says she's also already taken.

I guess this really depends on your definition.

If we're using a 40 bmi I hit it around the time I turned 18. Been on average over a 50 most the last decade.

If you're talking so out of shape I had to take a break climbing a flight of stairs I've hit it a couple times (lost some weight and gained back in a bit better condition) but the first was around 350.

How about getting winded after a short but quick walk across a road? Maybe 30 feet? That was new today, had it happen a couple times.

I want to be big enough that when I enter other people check the available seating to see if they should move.

Ran into a girl I knew in highschool today. Neither of us was ever thin, but back then I was about 250 and I'd guess her around 200. It's been 16 years since we'd seen each other. She looked a little shocked to see me in the big and tall shop she worked at. I was honestly glad she recognized me, were probably both pushing 400 now. But I had a lovely chat with her. Not often that I find old classmates who have fattened up to my level. Most the fat ones are just a hair over 300

Honestly, hard to say but probably happier. Lol 20ish years of gaining makes it hard to remember before, not to mention that I started young. But I'm bigger than I ever dreamed and I still enjoy it and want more.