I got myself a nice little linear rail from smalltec.de for 50 bucks:

This will now serve as the z-axis and also provide suspension so that the pen will have more or less constant pressure on the paper. The parts for attaching all the stuff were made from brass strips that I cut down to length, drilled and soldered together with a gas torch:

The large soft spring is for giving the pen pressure on the paper while the small stronger spring is attached to a mini-servo for lifting the pen and was plasticly stretched to allow some movement upwards when the pen is on the paper.
The finished assembly looks like this:

It works great and is very stiff, so much in fact that I noticed how bendy the 1m long arms are, but nevertheless v2 is a big improvement.