what to get

I know Int Moths are less of an ultra-lightweight's boat than they were a few years back but even so, sorry but you sound a bit big for a Moth. Rohan Veal, who ought to know, reckons the top weight for a Mothist is 80kg - see http://www.rohanveal.com/moth.html

Contender, on the other hand, could be a good match for you as you continue to grow.

At 75kgs you've got quite a choice. Top Vortex sailors are around that size (And looking at reports on this site, they average about 30 at the Nationals, 20 - 25 at series events). If you sail inland a Contender would be ok - and especially if you're growing.  Same applies to an MPS - but probably a bit outside your budget.

Best advice is to make a shortlist and then actually test sail all you're interested in.

Are there any suitable fleets near you? I.E. if there's a fleet of Contenders at your club go for that.

Of course, you'd probably be better off getting a Laser and sailing in good fleets until you've got all the skills and tactics needed and only then moving on to a trapeze class. Another possibility - crew for someone good in a trapeze boat - Fireball, 470, Hornet, etc - Some of the best helms were crews or have had a stint in the front end (and you might end up getting someone else to pay for the boat!)