Just adding an FYI in case anyone else finds this in a search. The cheapest and easiest repair here by far would be to replace only the trackpad (assuming the problem is with the trackpad itself), which is a relatively simple repair- remove back cover, battery, and then a few screws to get the old trackpad out. You can pull the key caps off the sticky keys, and clean the scissor mechanisms underneath which should get those keys to feel normal again. I don't think the backlight can be repaired however.
Replacing the upper case is a pretty tough repair, since all the components of the computer are basically fixed to that upper case, and must be removed step by step.