all 8th gen Intel CPU's have SmartCache. It's just a given now that it's what the CPU has since Intel won't use regular cache methods and haven't in a long time. If you click the ? next the cache on Intel Ark for the CPU you'll see that it refers to the cache as SmartCache.
As for the lettering, it refers to the level of features that the board supports. B is for business, H is for home and C and Q have to do with server level stuff usually. I would say to go with H for standard uses since some of the other features that are for business are going to be useless to you. Z is the top of the line and comes with the ability to overclock an unlocked CPU. You cannot overclock an unlocked CPU on any other chipset than the Z lineup.
Z series chipsets are a waste of money if you don't have an unlocked CPU. However, if you plan on upgrading to an unlocked CPU from the generation of CPU's that the board supports in the not so distant future then you can just invest into one now if you like so that you aren't buying two different boards in the long run. However, it might also be more financially beneficial to get a regular board for your regular CPU so that it becomes easier to sell as a combo if and when you upgrade to an unlocked processor.