Hi!
"i'd wait to do your security check until they actually want to employ you."
Jesski, if you haven't done your security check, they won't want to employ you, you won't get to that stage, they'll just choose people that have already done all of that stuff and are just waiting for the call. I would get it done.
Unfortunately the $$$ side of it is just a part of the whole process. It's just annoying that some groups that have gone through haven't had to pay a cent for medicals and security clearances while the next group has, but its just luck of the draw i guess.
Good luck!
Oz