Initially blasting them all on email maybe your best bet.
As you develop your business and figure who are really serious players then I would recommend you call them first and then blast the rest with email.
The thing is that there will only be a handful of people you will work with even if you have 243 people on your list. Those 10-20 investors are key to your wholesaling business, and its important to build relationships with those people. When you make a direct phone call to a serious investor, I think it goes a long way to building a solid relationship. If they are serious investors I will usually give them a certain short period of time (maybe a day at the most) before I blast it out to everyone. I also use that as leverage to get an answer quickly.
Friendships? Not necessarily. This is a business and friendship isn't your goal. A trust based business relationship is what you are shooting for. Now you will probably meet some people you can really relate to and friendships may develop, but I would never recommend trying to set up pseudo-friendships for the sake of the deal.