If you do ATPL theory and CPL practical, you will have a CPL. If you don't do the IR within the 3 years, you will have to do one of two things.
To get a CPL/IR without future chance to upgrade to ATPL you need to sit ALL of the IR exams again. I'm not sure how many this is, but I think it's either 7 or 9 exams under EASA.
To get a CPL/IR with future chance to upgrade to ATPL you would need to sit ALL of the ATPL exams again.
You would be better off to aim for the CPL/IR (SEP) if you are limited financially, since this at least locks the exams. You would then have to add the MEP and the ME/IR but at least you won't have to go through the exam pain again.