So, you are asking a dealer to tie up his money for several months until you are ready?? The dealer makes no money until you make your purchase. If I were a dealer, it would require a big deposit or full payment in advance. HOWEVER, that might depend on what options you want and how easy it will be for the dealer to sell it.
The other option would be to find the model you like and see if the dealer will/can have one in stock verses a special order. When you special order, you are adding features that others may or may not want or like.
