I'm buying my first bike and the LBS (not very local) offers free installation of accessories you purchase there. I'm not sure if I like their offerings -- although I suppose they're probably fine.

How difficult is it to self-install a little computer, lights, change out a saddle, and maybe add a rear rack and such? I may get fenders at some point, but not really ready for that yet, I guess. I'm not a gadget genius and I'm new at this, but I can follow directions.