yes, the bushes can be pressed out and replaced. You need a 10 ton hydrolic press and its important to have, or make, the correct drifts to put between the press and the bushes to get then out or they will easily get damaged when you put back in. There is a very small outside edge to press on, so its is not an easy job. I have Merle bushes in my wishbones and the car is now much better but there will always be some stretch in the rubber bushes and then the stearing so you will always have to stear into the camber/crown of the road a little bit. Before you press the bushes out, check the king pin/stearing knuckle on the wheel end of the wishbone is not worn first, it is usually ok. If worn you need new wishbones. You may find it easier to find a used set of wishbones from a breaker(car dismantler for used parts) with good steering knuckle and do the work on them before you take your car apart, because if the you can not re build the wishbones you will have no car until its fixed.
The knocking over bumps will probably be drop links (from anti roll bar to suspension strut/shock absorber). The knocking or cracking while turning the steering wheel may be the wishbones grinding on the subframe caused by very worn bushes. Hopefully that will go when the bushes are changed. Always change suspension parts in pairs for a matching set if from a breaker or for new.
Are you sure the oil on the power stearing pump is not coming from the cam cover seals which is directly above it? The breather system may be blocked. Is it definately power stearing fluid that is leaking?
I have searched for info on the merle(i "think" that is correct spelling?) bushes but i can not find it. I know its there somewhere i have seen it on here. Maybe some other members have the info you need. You will need a pair of drop links, a pair of large bushes and a pair of small bushes to give you a complete set for the car.
See also the guides in maintenance section for,
Cleaning breathers
Changing cam cover gaskits
Changing wishbones. It is important not to tighten the wishbone bolts until the car wheels are fully loaded and on the floor, then tighten as much as possible from under the car, you can then jack up the car again to give better access to tighten to 110nm and then angle tighten.
The drop links are easy, 20 mins. each side.
Hope this helps.