I would suggest your first decision has to be cadence or torque sensor, and this will depend on how bad your knee is. If it is painful on a permanent basis whenever pressure is applied to the pedals then as stated above yoy will probably be better with a cadence sensor, which allows you to 'ghost pedal' and as long as the pedals are turning the motor will propel you along. If the knee is not as severely injured as that and can usefully perform proper pedalling for a good number of miles, then a combination of cadence and torque sensors might be ideal. This will enable you to pedal normally with assistance proportional to the effort you are putting in, which feels more natural but with added oomph. The beauty of an additional cadence sensor is that should you meet a bad headwind or a severe hill you can twist the throttle and the cadence sensor will take over to drive the bike along as long as you ghost pedal. I have a Wisper 905 Torque which has both and although I have never needed to use the cadence sensor, I can imagine it could be a great help. Give Wisper a ring to check your model out. On their website the 705 is available with a torque pack upgrade available as an option. Re tyres, again Marathon Plus are excellent for puncture resistance, however my 905 came with Kenda tyres and I have put off replacing them as I've done 1300 puncture free miles on roads/cycle paths, and have no complaints about them. One improvement I made was to replace the saddle with a wide comfy one for £20, and install a Suntour NCX suspension seatpost at £50. The ride is now incredibly comfortable. Hope this helps.
