You don't need a MECIR rating to be an instructor.
The next bit is all in my opinion, other people will have differing opinions - take what I say or leave it as you see fit.
Doing an instructor rating (if taught well) should be like learning to fly all over again (it will make you think how little you knew) and will add good things to your flying. Do the rating with a school that has good, career instructors.
Don't do it just to build hours and think it is an easy way to get a flying job, do it because you enjoy flying and teaching and mentoring.
It doesn't pay well and many in the industry don't hold it in high esteem (particularly if you have low hours and go straight to instructing - not saying you shouldn't or won't do a good job, just you won't have much cred without wider or more experience).
It can be a personally rewarding journey in itself and can be a good step along your flying career but not if you are only doing it to get easy hours - in that case you will hate it, your students will hate it and your employer will want to be rid of you.
If you are only in it for hours, instructing can make you all bitter and cynical.
MECIR...
If you are working full time as an instructor it can be awkward to get time to do the MECIR if at a busy school.
Then once you have it and with minimal hours and a grade 3 FI rating you probably would not be giving any twin or instrument training anyway - that would go to instructors with more twin experience, not to low time instructors.
Doesn't mean you shouldn't do that training, grab it if you can (or can get it subsidised) just don't expect to get much twin/instrument time in your log book as a junior instructor with a brand new MECIR.
Instructing can be great fun and challenging but only if you see it as worthwhile in its own right and want to throw yourself into it. If it is just hours to get up the ladder - don't get a FIR, do as you planned, head north and look for other flying jobs. You will be happier.
NB I am not saying only do a FIR if you plan to make it your career - plenty of good instructors I knew, always planned to move on and did end up doing more 'glamorous' things once they had done their time instructing but while they were there they instructed with passion and professionalism.
As for getting a job as an instructor - much the same as any low time CPL - you have to be persistent, make a good impression and probably willing to travel. If you can find a good school with a passionate and experienced CFI, who loves instructing, that is gold. A good CFI is what makes a school.
And remember - pay is not good. Instructing is the bottom of the food chain. You do it because you want to.
Like I said, others will have different opinions.