I don't think there is an easy answer to that. It really depends on the condition of your bike and your preferences. I myself haven't modified anything except for adding a power outlet.
What you should definitely check are the brake pads, brake fluid, chain's tension and general condition, condition of the sprockets, age and condition of the tires.
What you should definitely check are the brake pads, brake fluid, chain's tension and general condition, condition of the sprockets, age and condition of the tires.