Thanks for letting us know what your needs are. Based on that, here are my recommendations:
i7, M295X, 8 GB RAM, and 3 TB Fusion Drive.
Thoughts and things to consider:
RAM. Get 8 GB of RAM now. You can buy two Apple OEM 4 GB sticks of RAM for $45 from the numerous people selling them from their new machines (myself and many others included). This will take you to 16 GB of RAM which should be plenty of RAM to carry you through the next 3-4 years. If you really want 32 GB for the rare occasion that you'll need more than 16 GB, then go ahead and buy two 16 GB sticks of Crucial memory now and either sell your Apple OEM RAM for $45 or hold on to it and put it in the closet. This is the cheapest route to 32 GB. (Versus having Apple ship it with 16 GB of RAM.)
3 TB Fusion Drive. This will give you the storage capacity you need to carry you through the next 3-5 years, assuming your requirements stay roughly the same. The Fusion Drive is fast...and it's fast enough for probably 95% of the users out there. This is the cheapest and most effective route. However, if you're doing multi-cam editing with several streams of HD or 4K video, then the speed of an external RAID array would be beneficial to you. If you know that you will invest in an external RAID array, then a 512 GB SSD is probably the better option over a Fusion Drive since storage space will no longer be an issue if you're installing an external RAID array.
In answer to your follow-up questions, if you want other computers to access an external drive, then a NAS drive is your best solution. NAS will not give you the speed that a RAID 0 or RAID 5 array will, but it will be an effective way to share the data amongst all of your devices. Personally, I wouldn't get a Drobo. Drobos are very easy to use (which is nice), but if the Drobo fails outside of the service period, you're pretty much up a creek without a paddle.
http://scottkelby.com/2012/im-done-with-drobo/
Hopefully this info helps. I'm sure it may generate more questions. Please don't hesitate to ask and I'll do my best to provide solid answers.
Cheers,
Bryan