Let me tell you what I did. 2 years ago I weighed 360lbs and today I weigh 170lbs. Like you I wanted to start riding with my boy when I taught him. All I knew was that I needed a strong bike, and I live where there is more dirt than pavement. I didn't want my 9 year old boy on the highway anyway, so I bought us both used Trek Mountain bikes off the local face book marketplace. $50 for me and $100 for him. I looked into sizing but was about clueless. I got him an extra small frame, and me a medium, which was close enough to ride.
What I really want to convey though is that I hadn't ridden enough to know what fit me and what didn't at that time, and as I lost 190 lbs,my needs changed. I bought another slightly larger $100 Trek 820 single track not long after that first bike that fit me better but I still needed a larger bike. Eventually I bought a Trek Marlin 5 in a large frame that I have ridden hundreds of miles for $250. Recently I bought a Surly LHT for $900 that only had 30 miles on it when I got it. I use the Trek off road and the LHT mostly on road.
I would probably look for a good used MTB if I were you for under $250 and start there. Now if you have plenty of money to avoid any possible bike service problems on a used bike then get one new or at least a used one from a bike shop that has been checked over. I just wouldn't spend a lot knowing that your needs will change as you learn what you like, and what a good fit feels like and changes as you lose weight.
Last edited by RH Clark; 04-23-20 at 12:07 PM.