I'm in the middle of converting my BF20L to CNC. It has been quite easy, I've been making mounts for ballnuts and motors for a few weekends now and plan finishing it this weekend. I also got my ballscrews which are premachined and ready to use, just have to repack the balls for bigger ones. I haven't really started building the mill but the preparation has been quite painless. Probably the electronics will give me few sleepless nights but thats ok if in the end I can make beautiful parts.
What I would say and others said to me that go with bigger than you intend at first point. Bigger have more mass, rigidity etc to make chips. And the bigger worktable also makes your workholding much easier, I know what I'm talking about. Also you don't have to upgrade so fast if you plan to grow, you can usually use the same tooling in even bigger machines because they have the same spindle.
Plan that tooling can easily take same money or even more than machine is worth! You will always find something that you need so this is a endless pit, sorry
Could you give me the link to that closed and open loop thing? Electronics is my weak point.
If you have more questions then ask