Joints like that have various lining materials between the ball and socket. Cheapo ones are usually nylon and don't last long, but proper racecar joints have teflon which is self-lubricating and should outlast the car in that environment. They can be fitted with rubber dust boots too to protect them, but there may not be space in the mechanism for that.
I'd be looking at 8 or 10mm joints which have static load ratings high enough to support the weight of the whole car. The reason for making them that large is because the linkages themselves would have to be pretty strong to resist the twisting moment which the motor will impose on them. The stock ones look flimsy, but because of their folded shape they are immensely strong in compression and horizontal bending. A 6mm joint would happily take the forces involved, but the corresponding link arm would not be strong enough (that's an engineer's opinion rather than anything calculated). A 10mm ID thin wall tube ought to be plenty though. It all depends on how much room there is in there, and its been a while since I had mine off the car.
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