We currently run ge with primary, secondary, and a historian server.

What would be good specs to have server wise running the dual premium server setup 10-25k tags and 15 thin clients. Thanks for the input. Any brand you prefer over another?

Have a look here: https://www.vtscada.com/scada-system-requirements/

While it's a great place for some recommendations, note that as a general rule, VTScada likes core speeds over the number of cores (ie: eight cores running at higher clock speeds is still probably better than sixteen cores total). Also, although VTScada history queries are typically at least 5X faster than competitors (My Stat is based on experience), An SSD is worth the extra cost to make things even snappier. Also, get yourself a good screen but things will probably be best running windows at 100% scaling so you also don't need to go crazy with resolution (I say this as I type on my 4K monitor with 125% scaling and use VTScada daily so, you do you but these are my recommendations ).

Non official response, but please don't even consider using HDD instead of SSD, the difference in performance is literally night and day.