Do i need a chain guide?
Probably. You can either run a bash (BBG are tidy and good value) and a N-gear jumpstop, or a chainguide (Superstar are fine, and cheap)
What is the best chain ring size?
32 or 34. You can get away with a 36 if you run the 11-36 cassette, or are strong, but with a 11-34 you’d be better off with a 32/34.
Any advantage with 10 speed over 9?
One more gear… the 10spd 11-36 is much lighter, as said.
Anyone running 1 x 9 for racing?
Not me but I run 1×9 on both my trail bikes (32t and 11-34, 9spd, Superstar guide, on both) and, for me, it’s ideal. On longer, tougher rides I sometimes miss a bail out gear but it’s a rare occurence. The simplicity and robustness outweighs any shortcomings.
I run a Renthal SR4 ring on one bike, which is lovely, and an old middleburn on t’other. FSA do a very cheap DH ring down to 34t (see CRC) which is a bargain and well made.