Hi Chuks
If your happy to spend money on relevant training, do your BOSIET and MIST. These are minimum quals for going offshore, don't worry too much about the medical as most companies, if not all, will insist on a pre-employment offshore medical anyway, so save your pennies on that.
As far as getting offshore in a safety role, it's not an easy path. you either need to be offshore already in some trade/professional capacity and then go sideways into safety or, which might be more pertinent to you, get an onshore job, SHEQ Advisor, or similar, which involves offshore travel for audits, etc. Then once you've got a taste of what the job is all about, you will be in a better position to apply for a full time rotational post. Give yourself a chance to see what life offshore is like first before going off the deep end is the best idea, it's not a life for everyone. Yes, the money's good, not what it was, but better than most, but that's for a reason. Two, three or four weeks at a time a long way from home, working, eating, sharing a cabin, etc. with the same (unchosen) people is not always easy to adapt to.
Some of the main training providers do starte packages geared to those who want to get offshore and you could save a bit on that.
Try Survivex, Falck Nutec, Petrofac and Maersk Training to see who's got a deal on these courses.
If you've the personality type that can survive it, then it can be a good lifestyle.
Hope this helps
Good luck