I'd seriously suggest you look outside Sweden. The market here is small so limited choice and high prices (Not helped by VAT 10% higher than, eg UK). Same is true of second hand stuff, low volumes, high prices.
Think you also need to be clear about what performance you need to achieve, how fast does your 'payload' need to move?, is 'point to point' positioning good enough, or do you need to follow a path?, what forces are involved - are you just moving the carriage or are you doing work with a cutter of some kind?
Something along the lines of the system shown above but using extruded aluminium profiles and ball bearing mounted carriage instead of wood and draw sliders would be pretty nice, structurally sound and eminantly scalable. There are commercial systems of this kind (eg Bosch Rexroth www.boschrexroth.co.uk - look under Linear Motion) but a DIY solution using whatever aluminium profile you can find cheap, and a bit of inginuity for the carriages is quite doable.
As far as control and positioning, there are some good answers above. Google "motion control" should find you more pointers. Also "DIY CNC" (Computer Numerical Control)
/david