"Not that it should influence your decision, but what does your custom combiner have for rows? if he is 6 row then I would look for a six row planter, if he is 8 then likewise"

My first thoughts exactly.  If you are going to have crops custom-harvested, match the planter size to the combine corn head.  Unless you plan to buy your own combine in the near future.   Then I would buy the planter size you want. 

Honestly, a 6-row planter will be plenty for 100 acres.    A 12-row - as Mike suggested above - is going to be overkill.

In my opinion.