I'd recommend against a singlespeed - if you're just starting to get in shape and you're complaining about knee pain you definitely should have gears for going into the wind and climbing hills.

Beyond that almost any hybrid or touring bike in your size should suit your needs nicely. I'd avoid any racing style road bike for now, but you might like a vintage steel touring bike, if you can find one on craigslist or at a garage sale.

Beyond that, it shouldn't be too hard to find a used bike that meets your needs for ~$300 or a new bike for about $600. (I'm think a bike that looks something like this Windsor Stratford.

I wouldn't worry too much about the brand or frame material - either aluminum or steel will work fine. Your local bike shops should have something similar to that bike that you can try out.