I agree with Chris, With that many setups, pants are going to be a nightmare. If you are set on doing a full manequin, then you can allway get some nice repro pants as a "stand in" for the time being. German field grey is expensive, and well faked. Make sure you get some opinions on whatever you want to buy.
--the 1918 should be fairly straightforward. The most expensive things will be the tunic, the cammo helmet (get a good one, it will totally bring out your display), and the grenades. The other equiptment wont be too bad.
--the 1914 will depend if you want a matching field grey cover to your pickelhaube, and what kind of unit you want. If you want standard prussian line, it will be costly but not prohibitive. If you want to go something like GdC, it will take a while and be expensive, very expensive (although it should hold a good value for you)
There are quite a lot of imperial collectors here. Get opinions on everything so you dont kick yourself later on. I went with what Chris did, full up torso displays with one or two full manequins. Fortunately for me, I collected American uniforms, to which pants are fairly easy to find (boots however...).