ILS30LGB,
No, you don't HAVE TO do your ME/IR FAA before converting to JAA. You can decide to take your IR skill test in a SE or ME when getting there. There is no point of taking your IR skill test in a SE since if you want a job you will need a ME IR.
ME flying in the UK is quite more expensive than US, you might want to learn to fly a ME airplane in the US and only do the minimum required for conversion. Also, having a ME IR FAA might help you in the future with potential employers (N-reg aircrafts).
Start studying your ATPLs first, it should take you 8-12 months depending on the amount of work that you are willing to put into it each week (10 months if you put ~15hrs/week).
You should plan ~$15k for conversion and roughly 10-12 mohths to get your fATPL in your hands.
good luck,
