I don't want to sound righteous, but how much is your health worth? I was in the same spot when I first starting mountain biking. I went in with a $500 budget. I'm about your size but taller and when I genuinely started seeing what my money would get me, I realized I would have to spend more and don't regret it one bit. I ended up getting a 04 Kona Coiler and rode the wheels off it. I don't regret it one bit and have loved riding for going close to 15 years now. It lets you relax, makes you feel healthy and accomplished. You can ride by yourself or with friends and will meet a lot of people. If you truly think you're interested in riding and it's not just a fad you think you want to try, find a way to at least get a decent bike to start. I guarantee you will enjoy riding more on a nicer bike (it doesn't have to be top end, just not generic bottom end). There truly is a difference, it's not hype.
The other thing to seriously consider, is that if you spend the money on a used nicer bike, say in the $1500 range and ride it for a year and decide it's not for you, you'll likely lose less money than spending $500 on a brand new bike and trying to unload it in a year. Not to mention the fact that like others have mentioned, you start with better parts, they are more serviceable and replaceable and you have a less likely chance of breaking stuff right off the bat. I can guarantee starting riding at over 200lbs you are going to be hard on a bike, with very little finesse that just comes with saddle time and have a high probability of breaking a cheap bike to the point of it not being usable or where repairing it is pointless due to the value of the bike.
(Financial advice aside and some may disagree, take with a grain of salt) IMO, even if you opened a 0% credit card and could afford to pay $100 for 18 months, you're better off budgeting $1800 for a new bike than plunking down $500 at once. And if you really want to crunch numbers, if you did buy an $1800 bike, rode it for 6 months and decide it isn't for you, you could still probably sell and be out the same amount of money....