Great response from @Ray Hurteau above. Couple points are worth repeating/expanding on.
1. Value - great point that the property is worth whatever it's worth today based on the current use/occupancy. This is like when you're buying a poorly managed multifamily. You don't base your purchase price on *potential* rents at the property, you have to price it based on the current rent roll. To the extent you are able increase the rents after you purchase it, that's value that YOU created and you don't want to pay the seller for that. As Ray points out, if the seller wants to stick to his number, make him come along for the ride - he'll get his price, but only if you get your approvals. You also save big money on carrying costs by structuring the deal this way.
2. Given that this is your first run at something like this, I would definitely partner with somebody on it. The best would be if there's another investor in town that you've seen have success with this type of petition - s/he'll already be plugged in to the town officials and professional service providers (attorney, architect and civil engineer) who can get it done for you. Next best would be an investor who has this type of experience in other towns - if you understand the mechanics of how this is done, you can find the right (connected) service providers and take it through the process in any town. I'd be careful about partnering directly with an architect or civil engineer - if they were good at the "big picture" stuff, they'd be doing it full time. Even some of the best of these guys (and gals) I've worked with are very good at their piece of it, but fall short of seeing the big picture.
Best of luck!