If you've got somewhere to store them then it's never too early to start preparing. My biggest issue about buying early would be a) you won't have a 'confirmed' number of guests yet so you may over/under prepare, and b) there's still 7 months to go where you might change your mind about what decs/favours you want and could be tempted to go with something else leaving you with a load of unused stuff.
In terms of space, depending on what you're having as favours, you could keep the majority of them 'flat packed' and build them up closer to the date. Anything we buy for the wedding but won't be needing until the day we've put in a box under our bed. It's pretty empty at the moment. Lol