Personally, I wouldn't buy a new holster and pouches for your Glock if you plan on switching to a Shadow. If you want to wait on getting the Shadow, that makes sense but I would recommend using what you had for IDPA until you're ready to make the switch. no reason to have a bunch of extra gear. I shot my first year of USPSA with extra mags in my back pocket and a cheap holster.
As far as which Shadow to get, I'd definitely go for the Optic Ready, the plate system works well as long as you get the right steel plates from CZ, and you can use the iron sights until you decide which dot you want to get.