The viper would be a good choice..The front bar can act as a pull up bar...Attach the platform section as low to the bottom of the tree as possible,and attach the seat portion about head high or higher. With the seat portion at head height you simply grasp the front bar of The Viper and pull yourself up and onto the platform...This allows you to step up on the platform with little bending at the knees.....Once you' re standing on the platform you must slide your feet into the bungi cord type sturrips which is the most strainious part for me ......I normally just squat (which works my knees pretty good) and pull up on each sturrip with my hand while sliding each foot into the sturrip. This may be uncomfortable for someone with bad knees.......

But I have an Idea! If you had a broom handle about 2 feet long with a smooth hook on the end, you could remain standing and use the hook to pull up on the sturrips while sliding your feet into them........This way you wouldn' t have to bend or squat at all..........I may try this myself!!! If this works I think you would be happy with a Summit Viper.......

Once you' re standing on the platform and your feet in the sturrips, I think you will have the hard part behind you......From this point to keep the stress on your legs and Knees to a minimum, position the seat portion down about knee or waist height and with stiff arms hanging by your side grasp the side bars of the Viper and support your body removing your weight from your feet.. Now all you' ll have to do is pull up with your feet bringing the platfom up ever how many inches you can stand before hurting your knees. You may have to take shorter strides than one with no knee problems......
I think the Viper will serve you well........It is a very comfortable stand......