Any currently-available iMac (in-built display) or Mac mini (with a suitable display monitor) that Apple is now selling can stream HD video and can handle email.
(One of the configurations I’m familiar with is a current Mac mini with several network-connected HD receivers, and that’s routinely recording four HD streams out to a disk array and entirely in parallel.)
If that’s all you’re doing, and if you don’t want or don’t need a big display, so too can an iPad.
An Intel i3 will work, here. (That won’t be able to push as much data as that earlier Mac mini, though.)
Check whether the Mac models you're looking at can be upgraded, as 8 GB is about as low as I’d go.
Biggest reason to go (somewhat) higher-spec with a Mac is for how long you plan to use this Mac, as user performance expectations, numbers and sizes of apps, and of data and photos and movies, all tend to increase over time. That, and some limits of some Mac models can’t be upgraded, or some others can’t easily be upgraded.
SSDs are wildly faster than hard disk storage, too. The two usual upgrades for an existing Mac—if the Mac supports upgrading one or both of these, and various Mac models don’t, or don’t easily—are more memory, and an SSD replacement for a hard disk.
iPad cannot be upgraded.
External storage can be added incrementally, too. This with USB-C and Thunderbolt 3, and with adapters to lower-spec I/O.
Visit an Apple Store, and have a look around. Chat with the Apple folks. See the sizes, and the alternatives.