There is a Radio Electronics magazine article a while ago describing this project (lawn mower).
If I am not wrong it use random cutting path and grass height sensors to search and destroy
I reckon it is better to find an allogarithm to cut the grass more efficiently than random movements,
plus en-cooperating solar energy for it, than to re invent the wheel.
This falls perfectly within the capability of a 16F877A.
However, if you have choice, you should try to use PIC18 instead.
Other than that, the idea of using a PIC is to make it to do what you want it to do..
An engineer should look and see what is needed and develop some engenious gadget to fulfil the need.
If you follows other's idea or project, then you are no more than a repair technician or a hack.
Good luck on your project and keep us posted on what you do.