a 4' x 8' CNC is no toy, so you don't want to half do it. Everything starts and ends with the frame, as it will be the thing that limits you the most out of everything. Trying to think of an analogy... A top fuel drag car. 10,000+ HP, transmission, clutch and diff to suit. Put it in the right chassis and it will run 3's, put that same combo in your street car and it won't go anywhere near that. While a top fuel car relies on flex, a CNC wants none of it to perform at its optimal performance.
Get the CNC chassis wrong and everything else right doesn't matter because it will be limited by the frame. For a 4' x 8' frame you don't want to be looking at extrusion for the frame (I do use it on my gantry though). Said it several times before but box and triangulate the heck out of it, that is what will provide you with a CNC that can cut more than MDF.
Motors... I started off with NEMA 23's but currently upgrading to a single NEMA 34, so I think you are in the right ball park there. For the long axis (X or Y depending on your country) I am using 3210 to stop the whip.
Spindle... I would recommend water cooled over air cooled.
If you plan to have a 4' x 8' cut area you need to make the CNC much bigger than the size of the sheet.
FYI if I was to remake my frame I would make a few alterations to it. Hindsight is 20 / 20. I can cut steel slowly with this CNC.