An IR is a rating, not a license. It's exactly the same as the IR you would attach to a CPL, so if you do your PPL first, then get an IR, when you subsequently do the CPL (which is reduced to 15hrs flying for an IR holder) it will automatically come back with the IR on it. If, during your PPL, you did a single-engine IR, you would have to retest in a multi as and when you required it.
IR exams do not credit towards ATPL exams (apart from IFR Comms, the 'joke' exam). They're a slightly different syllabus, more europe-centric. As far as numbers go the IR course has 7 exams and the ATPL has 14. Depends what you want your IR for really as to whether it's worthwhile potentially having to resit a load of almost identical exams. For most intents and purposes, an UK IMC rating is as good as an IR if you don't want to fly abroad or in Class A. The IR elite like to slag off the IMC rating, but it's hugely more accessible than the IR, lets you shoot approaches down to 200ft just like an IR and permits IFR and 3km SVFR in Class D. Plus it's valid for 2 years instead of 1. Whilst there are also rumours of its long term survival, it won't be disappearing for years to come yet.