The section on capacity planning in the docs gives a broad outline
The main driving resource would probably be memory. The rest is important as well but by the time you get a machine with 256gb of ram you're probably going to have CPU and storage covered.
As far as topology is concerned, the clustering is HA so I'm guessing you're going to want as fast an interconnect as possible.
We do not plan to use spatial or text search because we prefer to use it outside from stardog. We need tuning that stardog does not provide at the moment or are not clear how to do so.
Our experience with stardog and spatial search was not good, so we are doing it outside stardog.
Have you tried the spatial features of Stardog 5? We made some pretty substantial improvements (especially with respect to speed) over 4.x. In either case, what did you find it lacking that would have improved your experience?