Waiting those few months will cost you. The dealer should cut you a better deal if you get the BH as part of the initial purchase. He's going to be putting time into setting up the new tractor, and there is also labor in configuring it for a backhoe. If he has to turn wrenches on your tractor only once, it's a saving for him and you both. You probably can bundle the BH into the purchase financing, if any.
As for what brand of BH, there are good aftermarket models... Woods, Bush Hog, Wallenstein, Bradco, etc. The Kubota BH typically has some advantage because it is not a generic BH adapted to fit your tractor; it's factory-designed to fit your specific model. It will tuck into the back of the tractor better (which affects strength and performance), the subframe will be better integrated to the unit (giving better ground clearance), etc.