No direct experience with Sunrace BUT here's my thoughts on drivetrains...
1 - New cassette = new chain
2 - The spring in the X9 is strong and should be enough to keep the chain tight
3 - They will say you would ideally need a short or medium length rear mech, in my experience this isn't needed
What I would say is VWSurfbum is running a Praxis 11-40 I think with SRAM XO as a 1 x 10 and has no problems.
I'd also expect to see you change your front chainrings to a single narrow/wide chainring - this is normally a 30t to a 34t depending on how strong you feel your legs are - most from what I see are either 30t or 32t. As your subject is "1 x help please" I'd have expected you to mention the front chainring as well
For me something like a 32t Narrow/Wide Chainring coupled with the Sunrace MS3 and a new chain (personally a KMC ultralight works for me) would work spot on and the Sunrace cassette being £40 makes the 1 x 10 conversion dead cheap - especially considering the Praxis cassette it over double that!