@4F
Do you have to use a special fixed hub rear wheel for a fixed setup? Could I use a screw on cog on my old freewheel hubbed rear wheel?
A track hub has a dual threaded section, with the outer threads in the opposite direction for a lock ring. Your old wheel if only threaded on one side, won't take a lock ring. The wheels in the post above are good value @ £99, but might have to add spacers to make up the width from 120mm.
As long as you keep a rear brake, you can ride without a lock ring, but any severe back peddling could cause the sprocket to unwind.
Enjoy what you have done so far and look out for Project N+1
Well done
Keith