Since you want to replace the kingpins, replacing them "in/on the car" doesn't apply. The kingpins must come off, therefore the hub assy/steering knuckle must come off.

The upper pivot threads thru the top of the kingpin. Look at your replacement parts. The upper end of the kingpin has a threaded hole running crosswise thru the squareish top. The upper pivot goes thru that and has a couple of pieces; a threaded piece that threads into the aforementioned hole that is a fine tweak for caster, and an eccentric that goes thru a thru hole in the caster adjustment piece. This is a camber adjustment.

Go to http://www.diypartscatalog.com and supply make/year info for the car. I used a 1968 250S since this is what I had, but this setup was the same on all the W111 & W108 cars. Look in 'front axle/suspension' and you'll see pictures on the upper & lower , inner & outer pivots. This site has great pictures of the parts and their prices are pretty good. If you buy from there, try to stick to Febi & Lemforder for suspension parts. Meyle is NOT as good.

Regarding your question of using a spring compressor, these are generally used on 'strut' suspensions, which are totally different. There is just plainly no room to use any of these except the kind that have plates that slide in thru the side and then key into a screw with special 'dogs' to engage in the plates. The screw then goes thru the hole in the bottom of the lower control arm. Such a tool is necessary in some cases, and Mercedes makes available such a tool. It's something of a pain to use. It is used mostly for rear axle coil springs on the rear axle designs subsequent to the W108.

In the 108 front axle it is far easier and safer to drop those control arms and allow the springs to extend.

Study the parts pictures at diyautoparts along with your kingpin kit.

Might as well add new spring pads while your in there. These are quite cheap, and you'd be amazed how much difference replacing these old rubber parts makes in damping shock/vibration and you can't tell if the old ones are bad by feel. It's not like they crumble/crack in your hand, but what's important is that the shock damping quality has CHANGED over time. In the suspension, if it's old and it's rubber, replace it if it's not terribly expensive.