yes thats what you need. probably just the o-rings but for $15 thats not bad at all.
and yes a valve tool is basically just a 5/16 allen key with a hole drilled into it so it fits over the back of the valve stem. you can use one if you drill a hole in it...its pretty tricky to do it without it.
if your gun still has the plain hex shaped cocking knob you can take it out, remove the set screw in the back, flip it around, and use it for a valve tool....little cocker trick
. just be careful not to damage the threaded end with whatever tool your using to turn it....putting a couple nuts over it works pretty well to avoid that...put 2 on it, turn the inside one and it will tighten against the outer one, turning everything instead of just coming off.