The IMC or night rating won't count towards your PPL.
Doing the night rating is a good idea as night hours are the thing that some people have trouble with to get the professional licence issued.
In pure money terms it is not worth doing the IMC as the trainning will not count towards the IR, however if you can get an IMC course at a cheap price and stay in practice during your hour building it will no doubt make the IR less hectic and if you get a first time pass at the IR because of the extra IF practice it will have been worth doing.
What you have to balance with the IMC is the cost vs the cost of another IR test in a multi engine if you don't get a first time pass.
As you can see the IMC is a tricky call, my personal opinion would be to do the IMC as the IR is a big step to take all at once................. others on this forum may well disagree.