I hope your knee replacement goes well. Women I know seem to do better than men that I know after knee replacement which may be due to pain tolerance. I got a road bike for knee rehabilitation and a bledsoe brace. I swim a lot also. It may be best to to try out a few bikes at different shops to see what works for you. Starting out with a heavy duty "comfort" bike such as a cruiser or mountain bike may be a good start to even see if you enjoy it enough to pursue the idea of cycling for a better life. I have an aluminum Electra 7sp cruiser I got from REI that I have been using for many years. I do not necessarily get my heart rate up on this bike unless climbing, but it is great for getting my knee moving. I use the cruiser a lot when I am at San Francisco, Santa Cruz, and Monterey. I currently use the carbon road bike I got for knee rehabilitation for my daily commute to work.