The glue solution is great, but you do have to pick the right glue. IPS #4 is great for acrylics, because it makes a chemical bond, which is strong (
http://www.rplastics.com/weldon4.html). The wrong glue would be an issue. Screwing a barb into a plastic is likely going to lead to a crack. If you look at existing blocks, you'll find that many manufacturers that still use screwed on barbs, had to go to 1/2" thickness, or smarter, use a barb with a BSP thread (instead of the american NPT), because it's a straight thread, and not an angled one, which *I believe* makes a difference in the stress it puts on the plastic top.
The baseplate thickness: it depends a lot on the flow geometry. Usually, you need severely turbulent flow (like that caused by a jet) to be able to bring it down to 4mm or below, at least in the central area. The perimeter (of where the CPU core actually makes contact) isn't quite as relevant.
Roughness leads to higher surface area, but a thicker boundary layer. Again, depends on how turbulent the flow is.
Everything is a fine balance.