Hi,
I have had 6 knee operations over the years - all cartiledges, crutiates and a severed quads tendon, so not great knees!!!! I too was well overweight and on blood pressure pills - I was 60 yo. Any damp cold day, I took a half hour or so for my knees to get going.
My surgeon recommended I get an e-bike. I originally got a Kalkhoff impulse which I changed for a Haibike sDuro cross subsequently and can truthfully say that my health is changed beyond all my hopes. Where I live is hilly, very hilly! I live 5 miles from nearest town. 2 years later I have shed almost 3 stone, have got rid of the pills and my knees are transformed - no morning stiffness/pain and I can go up and down long flights of stairs or hills easily.
So don't get a bike to sit on and not pedal, get a pedelec and start with just a short distance on high power and build up. There is little need to put your knees under pressure at all with a good quality pedelec. Do check out which you can actually cycle NOW - you can then begin the process of changing your situation and staving off the replacement scenario !!!!!
Best of luck
Robbie Syme