Shop staff can be useless but one of the strongest bikes you can buy for reasonable money new is a halfords bike. The Carrera Subway. It's a strong mountain bike frame, rigid chromoly steel front forks and 36 spoke double wall 27.5" wheels. I think the maximum rider weight is stated as 136kg/300lbs but I believe there is an absolute maximum weight that should never be exceeded of 160kg. Pump the tires up slightly above the psi rating on the sidewalls. If it says 60 or 70 psi go for 65 or 75 etc. Just a tad more. Avoid pot holes and any sorts of drops until you have lost a bit of weight. Get a replacement saddle with a lot of padding, extended size.

Buy off Halfords online (preferably on a discount day) with home delivery rather than get assembled instore if you are going to find it difficult to collect or choose a Halfords store to collect with less childish staff.

Get out and enjoy the bike and your gradual increase in strength and reduction in weight. Try to keep off the carbs a bit more and limit treats to select days to accelerate the weight loss. If I could send moral support and encouragement to you through the internet I would as I'm sure many others would. It's a great way to extend you life and hopefully avoid health problems caused by obesity. I'm in the same boat as you. I'm about 144kg, 320lbs or something. I ride a Saracen jump bike. Built like a tank and looks like one. It wasn't dear, £40 secondhand although needed one or two minor fixes. This is the original image before I changed it a bit. You can see even the chainstays have reinforcing. It looks like it would be heavier than it is.