Rider weight limits regarding a bicycle purchase
I bought a Mongoose Dolomite as an experiment.
Bottom line - It's poorly made, assembled, and designed.
Here is what I learned.


Four inch tires have no benefit of any kind in normal riding. (At least the tires on this bike anyway.  I didn't try in sand or snow so maybe there is some value somewhere.).
I was hoping to find a tire pressure that would be super comfortable - roll over cracks and bumps.
I experimented with tire pressures ranging from the recommended pressure to just 5 pounds in 5 pound increments. At the recommended pressure the tires were so bouncy the ride was very rough. At 5 pounds the pressure was so low the tire wanted to crawl off the rim and steering was dangerous. There was no happy place it was always too bumpy or too soft.
The bikes parts are poor quality.
From the wet noodle cranks arms to the stove pipe steel frame there was nothing close to adequate in component quality.
Nothing was assembled correctly.
To get this bike adjusted correctly everything has to be lubricated and adjusted. Every bearing group (always too tight), every cable, every nut and bolt has to be checked and adjusted.
The gearing was wrong.
The stock chain ring was too large causing the gearing to be a higher range (too hard to pedal) than worked well on the bike. I replace the wet noodle crank arms with a slightly better crank arms that had a triple chain ring and only used the smallest chain ring. This allowed the bike to be ride-able.(Sorry, I don't remember the tooth numbers. I made a video about my experience - should I link to it?).  


Note: These are issues common to department store bikes.

If you enjoy attracting a lot of attention while riding your bike this is the bike to have. The fat tires drew a lot of attention.  

Finding the right bike
mikes has a good suggestion - a used older steel frame mountain bike with no suspension.  

Also, visit a local bike shop, more than one of them if possible. Be in "I'm only shopping" mode. Talk with them and see if they will let you test ride some bikes. This will help you form conclusions about what works for you.
There are many good bike shops that enjoy talking about bikes and helping people find the right bike for the task. If you find a shop that is high pressure or snobby just keep looking, good shops are out there.