Have a look through some of the build threads and all your questions will (OK not quite all) be answered.

Here's a good starting point http://www.mycncuk.com/threads/8242-...ght-ply-cutter

Then there's Jonathan's first build: http://www.mycncuk.com/threads/2288-...outer-building

Another couple of Matt's builds...

http://www.mycncuk.com/threads/5174-operation-cnc
http://www.mycncuk.com/threads/6530-...ine-in-a-month

Read the above build logs as a minimum, download yourself the free version of Sketchup and have a play drawing up frames with dimensions on etc and then post them up for critique. I'm not saying that the way they've built their routers is the correct but it's a proven route so can't be bad....