You can likely get a good bike in your price range on Craigslist (or some other similar site), but knowing what you're looking at is going to be tough without some experience. A cheap bike can turn expensive quickly if you end up buying someone else's problems. If you have a friend/colleague that has some expertise, and is willing to help that would be very beneficial. If you have some knee issues already, definitely get a bike with a triple chainring (3 front sprockets), it will help to minimize pressure on your knee joints, so long as keep your bike in a low gear. Also, browse these forums. There is a great deal of good information out there, and you'll read some threads that help as you navigate your way into cycling. Mosty, have fun. If it isn't fun, you aren't going to keep doing it. Keep your initial goals to a minimum other than to keep on keepin' on. Don't worry about speed, weight loss, etc. All of that will come so long as you stay with it. If you need a day off (from riding) take it, but don't let it become a trend. Let it slide a week and it'll be tough to get your momentum back. Good Luck.