How obese is too obese to ride a bike?
When your ankle flab gets caught on the chainrings.
OK, seriously...
Did you need a steel frame?
Steel is a wise choice for uber-clydes. It's strong, and tends to fail in a non-catastrophic way, if at all. Aluminum and Carbon Fiber have those catastrophic failures where everything seems fine, and then snaps apart. Steel gets creaky and weak feeling before it falls apart, so if you're worried about weight and frame failure, go with steel.
Was a wider seat necessary/more comfortable?
Your seat should be only wide enough to fit your sit bones.
Wider tires or narrow? Will too narrow tires go flat a lot?
Too narrow and you're going to flat them out from the pressure. Wider tires (28mm) will give a little more cushion to the overall ride and be more forgiving to the pressure they're taking. Check out loaded touring bikes. Many people use 32mm or even wider tires when touring with a full load.
Anything else to consider?
+1 to everyone that said check with your doctor before starting any exercise regimen. My fiancee just started a physical therapy / exercise program to get in better shape (doctor's orders, so she can qualify for lap-band surgery) and she was amazed at some of the initial metabolic findings, even though she felt fine. You never know what might be off with your blood gasses and glycemic balance.
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"I feel like my world was classier before I found cyclocross."
- Mandi M.