I think you should try and find a good deal on a used Leslie such as a 22H, 47, 45, one of the other single speed models that can be had for less. Make sure it has a horn, drum and (preferably tube) amp.
If you'd rather have a Motion Sound look on eBay. You can use a Pro3T with your Roland and it will sound very good.
I'll say, that if you plan to get a digital Hammond keyboard when you have the money (such as a VK7) the Motion Sound is the better option. If you plan to get a real B3, a Leslie is better. Basically if you play to be portable go with the Motion Sound otherwise the Leslie is better.
The Motion Sound hooks up with a 1/4" Input. The Leslie hooks up with a 6 pin cable. In both cases the M3 will have to be wired specially for it, and it will require a kit for the Leslie.