Experience be it PIC or DUAL etc does not expire, so it all counts towards licence experience requrements.
Regarding hour building, you need to consider a few things.
For the issue of a CPL you need 200 hours total, and at least 100 hrs PIC. You also need 150 hrs total before starting a CPL course, and the 70hr PIC is a requirement before applying for the MEP rating, but forget about this last one.
Basically, provided you do the CPL before the IR, it's good to have the 150 hrs total time requirement with nearly 100 as PIC when starting. The reason for this is, the CPL course is 25 hrs minimum dual, MEP 6 hrs dual, and IR is at least 15 hrs dual in the aircraft. This will being you up nicely to 196 ish total with no increase in PIC. In addition however, successfully passed skill tests go down as PIC/us, or P1s and not only count towards the 100 hr PIC required, but also the 200 hrs total time required obviously. Each test is in the region of 2hr plus change, so should tick both boxes.
This is how I did it anyway. Of course it's a bit of a balancing act, but it's easier to deal with when you know what the target is.