I was in the same position last year but I started by digging out my son's old Diamondback steel framed bike, the sort you can but for twenty or thirty quid and a turbo trainer. The problem was not just my weight but also my lack of balance. Cycling on the road was a definite hazard to my own, and others', well being. Set up permanently in the shed and with a radio, I used it 3 x 20 mins per day. Once I was sure that I would stick to the regime, I got a computer and Tacx trainer so I could cycle with the films and virtual reality. When the weather got better, I bought a Trek frame from £23, resprayed it and added wheels and gears from
ebay and started cycling along the traffic free greenways. This, being an aluminium frame did not take my weight easily. If I set the gears on the stand, my weight distorted the frame and caused the chain to rub on the derailleur in certain gear combinations. It's not really a problem because it was all cheaply done but when I eventually bought a well built lightweight Scott mountain bike, I was a bit cheesed off to read that I was over its weight limit. I got it at a bargain price and I have had no problems using it, but various bearings are going to wear more quickly. But, all the gears are perfect. I then got a touring bike, heavier and subtantially built and so far so good.
At your weight you have to treat your bike with respect and not stress certain parts. Don't jump out of the saddle and start slamming down on the pedals for example. You don't know what is going to buckle, or bend, or break. You don't want to be leaning too far forward with lots of weight on the handlebars because if you slam on the brake, something may fail around the stem or headset or on the brake itself. A nice upright position, a more traditional type of setup, is probably best and a steel frame.
Fortunately, there are quite a few of these sorts of bikes at very good prices 2nd hand. They will give you good service and are value for money.