Notwithstanding the legality of opening a PPL with a MEP Class Rating, I would concur absolutely with the advice given above. You would have to undergo far more theoretical training, expensive flying training at 'commercial' rates - all for a licence which wouldn't allow you to fly a SEP aeroplane! FTOs require approval to conduct MEP training since the advent of JAR-FCL - it's nothing like as simple as it was in the days of the PPL with a Group 'B' rating.
It would be hazardous to attempt to run before you can walk. I would suggest that you obtain a PPL with SEP Class Rating first, build hours and experience, then learn the rudiments of RG and VP on a 'complex single', then do a MEP Class Rating. But only bother with a MEP Class Rating if you either intend to use it routinely or wish to become familiar with the aeroplane before moving on to CPL/IR. You don't have to do your IR on a MEP for a CPL/IR; however, many people do as a buidling block towards ATPL/IR.