No one else has mentioned, so I'll chime in.
Are you certain you want to add a Multi to your PPL before you have your instrument ticket? I did this personally, and regret it. It was simply a waste of money.
If you plan on continuing your training, presumably your IR rating would be next in line which I can logically assume will be done in a single (Unless of course you ave deeeeeep pockets).
Anyway, if you get your PPL-ME add on on, and then get your PPL-SE IR, you will have to go back and take another checkride in a Multi in order to have the "Single Approach Only" Limitation removed. On the other hand, if you got your PPL IR first when you went to add your ME rating you only have to do an extra single approach and you have your PPL-ME IR free and clear.
It will save you money in the end by getting your PPL IR first, and quite honestly what are you going to do with a PPL-ME anyway? No one is going to rent you a multi, so it is really just bragging rights.
Food for thought. Good Luck.