Did the same; no real dramas for me either; however, make sure you do it either during or after your ATPL study, NOT before - otherwise you may just have to renew it before taking your ME/IR skill test (MEP[L] validity is one year).
If anything, it made my life easier for the CPL (I had tasted a complex twin before getting to grips with a complex single!)
I would recommend either the Piper Seminole or the Beech Duchess (easiest MEP types to learn on), but you may want to do your MEP rating on whatever aircraft you'll use for the ME/IR.
Cheers
