It's a tough call and I'd certainly not want to do anyone out of business, especially in these times.....Thanks Gordon for putting even more duty on our petrol.
Knowing what I know now and the fact that you have time to research here, I would take the route of getting the various parts myself and do my own thing. Answering your questions:
1. That one is a real search the forum and weight up what people say. Arguments about reduced backlash due to no coupler flex etc. Another good reason is the fact it can make mounting the motors easier. Also the Syil kit has a bottom mounted Z axis motor and that looks like they take a pick axe to the base casting to relieve it to allow the motor to fit. Unless this is the way all X3 bases look!
2. Not a very common thing to do with ball screws and I'd say to make it work you'd need to go down the motor/pulley drive route. I'm not sure whether the X4 mill with MPG can drive the axis regardless of whether any control software is running (Mach3 etc), but you can obviously use an MPG to control movement via Mach. I use the Shuttle Pro plug-in as a cheap alternative to a quality MPG. People also seem to like the Xbox controller plug-in.
3. The Syil kit is open loop.
4. The closed loop by encoders or scales would have to be handled by the controller software. Another area to research here and the Mach support forum. So far I've not really encountered an issue with it being open loop. I find my brain has more control issues than anything when it comes to doing daft things whilst using the mill.
5. The drives with the Syil kit seem to be okay, but the other boards are mediocre to say the least. You'd do better with an established third party breakout/driver board. The Syil interface board will handle four axis and has a relay for coolant, but is possibly a bit limited on IO.
No doubting, besides the very poor spindle control, the Syil kit will give you a functioning CNC mill.