I was wondering if this would be too much of a workload for the Mac or if this is a normal thing to do with such a device.
What is "normal" is very subjective. What you actually intend to do with your Mac is what determines if it's capable of handling the processing load.
This model is capable of handling two external displays up to 4K each but that doesn't mean you're going to be driving high intensity graphics apps like gaming or video production with this configuration.
- There's no discrete GPU meaning it utilizes your CPU display chipset and up to 1.5GB of system RAM for video memory
- There's only one Thunderbolt chip so, your video bandwidth is limited.
Also, just because a computer can do something (it has the capability) doesn't mean it's sized appropriately for doing that thing long term. Example: just because you have the ability to plug in two display's to your MacBook Pro without a discrete GPU doesn't mean you that you've sized it right - you probably should have purchased the 15" with 4 Thunderbolt port model with Radeon GPU. It's even possible you should have gone with a desktop.
I was looking at either buying a Henge dock or a cheaper option [USB Hub] .... Does the adapter make a difference for this too?
Yes! That device you linked to is a USB Hub. Meaning, all the benefits of Thunderbolt will go unused. If you want to get the best possible performance out of your machine, you need to use the components that will make the best possible use out of the resources you have. Get a proper Thunderbolt Dock instead.
The Henge dock, does nothing more than take your TB ports and move them so you can have the Mac "in an upright and closed position." Other than being slick, it gives you no audio, video, Ethernet, USB...nothing. IMO, it's a waste of money.