Sorry, I'm not really sure what you mean by all the quotations being question marks???
You've mentioned that you intend to use 1/8" endmill...in case all the channels are 3/8" minimum, than larger endmill is an option, meaning shorter machining times.
I'm so glad that I've guessed that you intend to make a CPU block...
...I made a few myself, but for water only. However, you mention phase-exchange type of block...have you ever had a chance to take look at the inside of the Vapochill or similar block? I haven't, but I'm not sure that it should look like this...that kind of block is supposed to be a vaporizer, the place where compressed (liquid) medium (freon or similar) expands into gas with a significant temperature drop (I am sure that you know the theory, but just want to mention that). I am not sure whether there should be a channel in the block with enlargement of cross-section surface along the direction of medium flow inside...I also don't know whether you are going to need some capillary tube entering it or not. If you've already taken care about everything, sorry for mentioning this.
IGS, STP, DWG and DXF are pretty much standard formats...IGS and STP are 3D and the other two are 2D. I would prefer the firs two mentioned, of course...
Regards