I say 10 speed as I have a 9 speed road bike and replacment parts have been tough to find cheap. I've seen a lot of used 10 speed stuff cheaper than 9 speed.
That said 9 speed MTB stuff is easier to come across. Maybe ill look into that.
I am a pro at eBay and craigslist scouring. I don't plan on buying much if anything new for this build.