He already stated that his Farseer is on foot, so try to concentrate on using him accordingly.
Runes of Warding are solid for enemy psycher control, so I liek to take them.
Runes of Witness on the other hand, are good to ensure you get your Power off; but they add significan trisk for Perils of the Warp; thus I just don't take them.
Spirit Stones These are very user specific; but in general, if I can't use both of the powers that go with them for at least 3/6 turns, then the stones are not worth their cost. So having two powers that go off in teh shooting phase is not a good idea. They are best suited to powers that have a good area of effect, and go off seperately such as E-storm, and Doom or E-storm and Fortune, or E-storm and Guide. Just make sure that your going to take the right skills to complement your force well. (show us your list - or concept at least - and we can better asssit you).
Singing Spear Good for anti-tank; but useless if your hanging back. So again think about how your farseer will be fielded on the table.
Powers. All of them are army dependant as well as working with your playstyle. But here are some pointers.
Fortune Best used on larger squads with a save that is difficult to remove. Wraithgaurd, Concealed Guardians, things with Inv saves.
Guide Best used on units with a lot of shots and not so good Balistic Skill. Guardians, Warwalkers, etc...
Doom Best used when your army has a lot of low Str shooting or attacks. If your fieldin an army with 100 Str=6 shots, then Doom is near useless.
Mind-War I love this one; but it is best used with a Jetbike to get you close enough to actually get it off. Its limited range and LOS requirement can seriously hurt a footslogger.
E-storm Good for anti-tank and horde use; but it must be supported by the rest of you army, if your have limited shooting, then its near useless.
I hope that helps you out.