It depends on the class. The class number is almost more important than the size (as long as it’s at least 8gb) or whatever ‘ultra’ or ‘mega’ or ‘fast’ name the vendors give the card. Here’s a decent story from How to Geek that might help:
The one from the OSMC store will 100% be what you need, but you can use (or buy) something else too. I have a class 10 in my RPi3 and have no problems at all.
You just need to know that all Class 10 cards are not created equal. The speed of random 4k reads are what is important to Rpi/Kodi. It also might not be outside of the realm of possibility that some Class 8 card might have faster random 4k reads than another Class 10. I have no experience with the SandiskUltra, so I can’t recommend.