Great question. First, completely field strip your rifle as if cleaning it and look at the loose parts you have, like the piston and piston spring, for instance. Could they be lost if you cleaned your rifle in a field or a messy garage? If so, you may want spares in your kit.
This goes for the locking piece on the bolt too. It just sits on top and could be lost.
Look at this YouYube video for cleaning the bolt: http://www.czforumsite.info/index.php?topic=31691.0
Those small parts can wear out and become lost if you disassemble the bolt. I recommend that you have spares around if you take your bolt apart, in case you lose something, which would disable your weapon. Or you could just buy a complete assembled bolt to have as a spare for $50. The extractor and firing pin probably get the most wear in the bolt.
While you’re at it, get a spare striker. It affects the firing pin greatly, and can become lost or worn.
I had some problems with the bolt hold open feature working with all of my mags, so I would get a spare bolt catch and bolt catch spring, as well as some new mag followers, as both areas should be addressed. I think the bolt catch may wear down with use and followers wear down too. A spare magazine catch spring might help lock mags in tighter too, so the BHO can work well.
The obvious thing is springs. They go in the bolt carrier, FCG (disconnector) and other areas. You would really have to shoot a lot to wear them out though, but when they do fail, it will really mess up the function of your rifle. A spare magazine spring is cheap insurance too. I’m a long way from wearing mine out, but I have some spares for when I do.
You can get all of these parts from CzechPoint. Obiwanbonjovi also sells them reasonably here, including U.S. pistons, which count as one compliance part. Some parts can be found on gunbroker and ebay, but shipping is usually very high. You cannot get all of them in a VZ58 armorers chest, but you can get most of them there. You can probably get everything you need for under $100.