Most of it is powdercoated,and i am thinking of putting a clear coating on the new ball joints,tie rods,pitman arm. Will this last?

Any clear coat out of a rattle can won't keep the moisture out and you'll end up with rust under the clear in no time. I would go with a high solids urethane paint instead.

What about just wiping them down with a little oil once in a while?

That's probably your best bet...I wipe my bare/plated parts down with a light coat of oil and it works as long as you leave it on there and not wipe too much back off...Yes, it may get dusty or road grime but it will wipe off and it's better than rust...Clearcoated parts look terrible when they start to rust under the clear...It "spiderwebs" under the clear and gives the part an unnatural appearance...