Ezz,
If you get your MECIR, keeping it 'current' is worthless and expensive. All you are doing is paying for twin hours. Realistically, do your renewal every year (mine used to cost 1 hour of aircraft and 1 one hour of Sim + ATO fee) and wait for that twin job. The operator will no doubt do some sort of ICUS (remember you need 10hours on type to operate IFR CHTR) so all of your currency will be fixed up then.
As far as which aircraft to do your MECIR on...I asked the same question 13 years ago. I chose the C310 because I was convinced that the C310 was the endorsement of choice. Result....never flew one again. First twin job was on a Baron.
As to whether you should get it now or later my opinion is this. If you are aiming for a career as a professional pilot and have aspirations for bigger and better things than a 210/206, get EVERYTHING done ASAP (ATPLs, MECIR etc). You never know what opportunity is around the corner. I have seen many guys over the years miss the opportunity to take a twin job offered to them because they didnt have their MECIR. Likewise, many have missed the boat with airlines (from regionals to majors) and turbine operators because the ATPLs were not done. My point is, whether you have 500 hours or 5000 hrs, you never know what opportunity is around the corner.
And finally, having the MECIR gave me a little extra flying in a higher performance aircraft which boosted the confidence and made the transition from the mighty PA28R to the C210 a piece of p*ss.