They don't need to be personalized, but that is a long time for to keep M&Ms. I would check the expiration dates on the bags and read up on how to store them.
You can order personalized M&Ms online, but they are actually pretty expensive. I think you can also order them in specific custom colors for quite a bit less (but still more than regular M&Ms). If you want to get pink/burgundy I would look at the cost to order those colors online. I wouldn't want to buy them more than about 6-8 weeks before the wedding....
Personalizing the MMS themselves can be expensive. You can personalized the container you put them in. If you use a bag you can get a personalized tag or use a container and add a personalized sticker.
I wouldn't buy M&M's on in Feb for a September wedding. The personalized ones are outrageously expensive so I would just go for the colored ones which you can order.
Just wanna share my experience with the personalized m&m, me and my fiancé used them for our engagement party. It was def a case of buyer beware we used the one that came wrapped in the white bags with the doves on them. They are definitely cute but very small. No exaggeration there are about 15 m&m's in a bag. I received the order a 2 weeks before the party and freaked out when I saw how small they were. That night I went online ended up ordering glass heart jars and the went to party city because I had to buy additional candy to put in the jars with the m&m's.
As far as storing them I would def order them closer to the wedding. Our party was in may and it was hot so they started to melt we put them in the refrigerator and once they hit the heat it was the same issue, so be careful when storing them
All and all I have to say they are good if you are doing a small intimate party but if you are doing them for a large party it is going to get costly.
I added pictures of the finished favor in my profile.
If your main concern is affordability, and color coordinating, I suggest you look for the candy "Sixlets". They are available for bulk orders on line, come in a variety of colors, and are much more budget friendly than M & M's.
Re: MMS favors
Websites/blogs where our wedding has been featured:
http://www.dapperq.com/2013/11/a-very-dapper-wedding/
http://www.onabicyclebuiltfortwo.com/2013/10/wedding-christina-g.html
http://4realequalityweddings.com/2014/05/16/g-christina/