I designed the machine on Sketchup which enabled me to determine the lengths for the ball screws and rails but I did not start to build until they arrived because then, as you say, I built the machine around them. Having sad that the items arrived exactly as I ordered. You could therefore buy a generic set from ebay and design the machine around those.
Yes I paid VAT and import duty on the amount I was invoiced for, the bill came from the courier in this country.