ISAF employs may prefer Ex-MIl personnel but it is not compulsory.
The security clearance can be a pain (has to be done in London), but once out there the job is not much different from what you do now.
I know a couple of peeps who have worked for ISAF including 1 ex SGS/SWP from MAN who worked out there for 3-4 yrs doing check in. She did not have any mil experience btw.
Good Luck and if you get it keep yer head down
PS also have in mind that days are numbered for ISAF in Afganistan with the withdrawal coming up shortly....