Hi Ron -
Sorry for late response - have been out of pocket...
When I was looking at CNCs, I considered everything from build-your-own to the very high end turn-key systems. I'm an electrical engineer with extensive motion hardware and software background and my conclusion was - buy a solid all-in-one machine and start cutting wood! There are a lot of intricacies between the hardware and the software that need to be considered. I dont know anything about Zenbot, but strongly recommend a turnkey system. People have spent years tweaking the hardware/software interface and getting everything just right. There are a number of decent CNCs out there, from Shopbot to Axiom to $$$.
Consider how accurate and maintenance free you want it to be - I chose a system with acme screws over belts and pulleys. Consider how much you will run it continuously - you can get a head with either a standard router mount or a spindle motor/ I chose a water cooled spindle motor and love it.
As for size, consider what you want to do on a regular basis - I chose a 24 x 36 because that's the vast majority of what I need to do (BTW, I've used it to surface plane a 24 x 60" glue-up using the right strategy. You dont have to be limited to 24 x 36)). For the one project in the last 5 years that needed a few larger parts, I shopped it out. Dont forget that the floor space is considerably larger than the bed dimensions - the machine needs extra room for the gantry movement and you need to get to at least 3 of the sides for loading, maintenance, etc.
There's a lot to consider - and as the old adage goes "get the best tool you can (almost) afford at the time".
PM me if you'd like to get into more details on my personal decision process.