Coil overs won't fix any of those things.
There are many things one can do with an 850, re-aching the springs, reversing the spring eye on the spring, using a lowering block for the mount of the spring to the body, weld a new lower mount onto the kingpin upright to attach the spring eye to, replace the leaf spring assembly with a new lower a arm with mounting and with this adding coil overs.
The things you are mentioning are all resolvable with appropriate alignment and suspension set up, a spacer on the steering arm might be used to correct for bump steer, one could also look into changing to a rack and pinion steering set up even.
To go to coil overs one would want to reinforce the upper shock mounts and one should likely think about the upper A arms as well. One might also think of changing the steering knuckle to a 124 type so you could use ball joints rather than the king pins in the current knuckles. The possibilities are endless
I would look at re arching the spring, replacing the shocks, replacing all rubber parts and all steering parts which would include rebuilding the idler with new bushings.
