As its the International Special Training Centre, I think the first qual is SF. After that I'm sure its a basic instruction course.
edit; I believe some courses are also open to LRRP, Recon or Scout troops>
Thatīs not really the truth, you donīt have to be SF to take a course there. Every country has itīs own rules when it comes to sending troopes to the ISTC. Belgium has about 60 something qualified SF-operators, so they also send paraīs, recce or just infantry down there. The USArmy even send tankers down there to take part in the sniper course.
And itīs not just open for army troops, seals and marines from different countryīs have been seen there as wel.