Here's how I'd go about it.
Decide whether you want to use 2.5" or 3.5" drives. If you're looking for portability, and/or if you don't want to be plugging your drive into the AC outlet, then you'll be looking at 2.5" setups. But if you want high capacity and high speed at high capacity, then you go 3.5".
Example. The largest-capacity 2.5" drive I know about is 1 TB, but if you want it to spin at 7200 RPM you're going to pay well over $200 for it. The last time I bought a 750 gig 7200 2.5" drive it was something like $140.
With a 3.5" drive there are many to choose from and they are all well under $100 for 7200 rpm.
For a given speed and capacity, a 2.5" drive is going to cost you more. And as far as I know, you'll be capped at 1 TB.
Oyen Digital has 2.5" enclosures, and I've used two of them and they are just fine. $60 or so for a USB3 / FW800 enclosure. They aren't the only manufacturer, but I've used them so I'm mentioning them.
You also have to decide on capacity. As above, if you go 2.5" you're limited. If you go 3.5" then you can have anything up to 4 TB. In practice there are a lot of 1-2 TB 7200 rpm drives out there and you can buy by price or manufacturer, as you prefer.
With a 3.5" enclosure you have a lot to choose from. Lately I've been using a Macally NSA S350U3 ($50) and a Seagate 3 TB drive (about $120). The Macally is USB3 only -- no Firewire. Other companies make USB3 drive enclosures, of course. I've used Macally stuff for a few years and it's just fine.
So you could get the Macally for $50, choose a 1 TB drive for maybe $80, or a 2 TB drive for maybe $100, or the 3 TB.
It's up to you. Just remember that a 2.5" solution is going to be more expensive than an equivalent 3.5" solution.
It might be that you want an external you can stick in your pocket. But if you want capacity over 1 TB there is no 2.5" solution at all (someone will correct me if I'm wrong).
What I meant by "in the long run" is this. Suppose you do a 3.5" 1 TB setup, which will cost you around $130. But then, as your video storage needs go up and up, you need 2 TB or even 3 TB. Take out the 1 TB drive and replace it. It's still good, so you can slap it in a cheap enclosure or get a SATA-USB emergency adapter, and then you can use that drive if and when you need it.