I'm roughly the same size, 6'1" 185lbs. I would recommend going to the local bike stores and test riding some folders if you can. Some ride/feel very differently than others when it comes to bigger people mashing on the pedals.

In your price range of under $500, I would go for a used off craigslist Xootr Swift, Dahon Mu or Dahon Speed, as they ride nice and are pretty stiff feeling which is what us bigger guys need. The cheaper bikes such as Citizen or cheap ebay brands, either flex too much (poorly built for big guys) or are too small. The ones I rode, felt like I could easily rip the handlebar off if not careful. Same goes for the cheaper Dahon designs such as Boardwalk/Mariner/Eco, all felt too flimsy and small to me.

As for new, I've read good things about Downtube folders, which are in your price range. But never ridden one so don't know for sure. Dahon Speed Uno is a nice cheap bike if you are ok with single speed. But you will have to add a front brake as it doesn't come with one.

Forget about Bike Fridays/Bromtons/Birdys as they are double what you are looking to spend. Even used they are $7-800+.

But don't take anyone's online word on whats best, go out and test ride some bikes and see for yourself.