What sort of security? If they are going to lend you the sharp end of 50k then they are going to want something pretty meaty.
Although 18 is the right time to do it beofre you fall in love etc, I don't see any job being worth debtsville. Your choice, your move and I hope it pays off. However, why not apply for mil selection? You might be lucky and get it all for free - and commission training will develop you as a person