I own the Waves SQ a card, and a server (along with cards for other consoles).
It’s a tough question to give a straight answer because it’s so dependent on how comprehensive your plugin needs are.
The “Impact Server” will suit most end users. But if you plan on off-loading all of your reverbs, delays, etc to MultiRack SoundGrid, you may want to go higher as those are quite CPU intensive.
You can also drop your MultiRack SoundGrid sample rate down to 48k which will give you more plugins since your CPU usage drops almost half.
Latency is of no issue with any hardware SoundGrid setup.