I'm in exactly the same position as you are in that I want an 8x4 router for full sheets.
Looking around I decided to go with a Joes CNC hybrid design- comes in around the 2-3K price or more! ,all upgradable from the basic model which starts out with a mix of materials -MDF ,HDPE ali and steel , right up to his new design which is called the evolution 2.0 which is all made from 20mm plate aluminium.
It is a paid for plans/forum which I have bought but you do get access then to all the upgrades available.
I have my base made now and just waiting on my aluminium profile rails to get started on the actual machine.
I will post up my build on here in case anyone would be interested.