Just take into account that I've already owned the Mini3D and my calibration equipment for 2 years: If you buy one now, when you replace your projector for a 4K one in 3 years time, the Mini3D is only 1080p so would need to be replaced for a 4K path. Also, unless you are going to calibrate then the Mini3D isn't going to be much help, so you have to take that into account as well.
The Darbee is worth buying anyway IMHO and cheap enough that it doesn't really matter what it's worth in 3 years time. FWIW you can use a Darbee with an X55 as well, so it's more of a universal accessory.
When you say that your room is black, if you mean the walls and ceiling then fair enough, but if you just mean that you can make it dark, but your walls are light then that is my current situation, so you won't get the best out of it. I've seen an uncalibrated X35 in a really good all black room and it looked way better than mine purely down to the room.
Zooming: Been there, done that, bought the lens...