Based on what I've heard, I'd say the realistic bare minimum for any kind of MDS is probably 8 cores and 16GB RAM. Beyond that it really depends on the number of CMAs (and associated processes) and the size of the configuration (number of rules, objects, policy packages, etc.). In general though:
More CMAs -> More Cores.
Larger Configs -> More Memory.
A Smart-1 3150 has 12 cores and has a base of 64GB RAM; a Smart-1 5050 has 16 cores and a base of 64GB RAM (supports up to 50 domains & 50 gateways according to the pricelist); a Smart-1 5150 has 24 cores and a base of 128GB RAM (supports up to 100 domains and 150+ gateways). For an MDS if you have to pick between more cores and more RAM due to budget, I'd vote more RAM to help buffer disk I/O operations which tends to be the most prevalent bottleneck when it comes to MDS management performance.
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