You need to read... ALL OF THE BUILD LOGS. from start to finish. You'll need some strong coffee.

Dont take anything I say as gospel, as im just learning. but you'll see for yourself designing it is key. You want to draw a proper 3d model. for this download google sketchip 'aka Trimble' its free and there are plenty of tutorials all over the internet.

When your reading the build logs you'll see the amount of thought that goes into designing against 'flex' or 'deflection' and 'vibration' all of these factors need a lot of thinking about right from the start of your design. after you have seen a few designs evolve from the original concept ideas the following versions to the final versions you can see how the strength of the design improves.

Worry last about the electronics, as its only after you have made a design to suit your requirements that a guage on the size of electronics is possible.

for cheaper gear, with less support, chai is reliable http://stores.ebay.co.uk/linearmotionbearings
for better service, with good support, you want zapp Zapp Automation Ltd

good luck with your build. Youtube is a good source of videos :D