If you want a street legal bike I would look at the DR400 for a guy your size. There are a lot of them around in the used market, they've proven to be pretty reliable. However, there are a lot of good bikes out there in the used market. I would shy away from the 650's as your first bike, they're typically a little heavier. And a lighter bike would serve you better on the trail. I think a 250 would be a little bit small for your size and you'll be wanting more power in short order. There are lots of bikes that'll fit the bill for you, in your price range look for older Suzuki DR400's. I also think you can find nice Yamaha XT350's and DR350's for even cheaper than you've budgeted. If you have friends that have bikes similar to what your looking at, talk them into letting you take a spin on their bike. A quick spin will tell you a lot about what you want in a bike.
On a side note, the KDX 220 you were looking at is not a good bike for you seeing how your just starting out. It's a two stroke and produces power completely different than a 4 stroke. A 4 stroke produces nice trackable linear power which will be much easier to learn on. A 2 stroke produces more explosive power which is IMO more exciting and fun to ride. However, 2 stroke bikes are very unforgiving and it will kick your but in a heart beat if your new to bikes.
Hope this helps.