Just accumulate things as you need 'em. Not necessary to go to huge expense right away.
Most of the maintenance you need to do can be done with the took kit under the seat, believe it or not, including changing the spark plugs.
Just do NOT over tighten anything.
I was just thinking about this the other day as I removed the front sprocket cover with a 20-year old set of sockets and wrenches bought on special at the then local Chief Auto Parts store. The thought occurred to me that I should get a good set of Craftsman or Snap-On tools. Since I'm not working on 20 bikes a day or anything, however it seems pointless. I'll stick with my little tool set and buy what I need as I go.
One of the first things you should get, though is a torque wrench and a socket set with a three or four inch extension.
You'll know what you need when you get there.
--Wag--