Q1: The SP3 should include a stylus, and so that should prevent the finger spraining issues you'd have playing on a touchscreen with your raw finger.
Actually it's deals where you need to enter cash values that I'm not sure what you'd do without a keyboard, but the SP3 should come with that. (You'd also need to lean the gesture for zooming out if you didn't have a keyboard unless you like how far zoomed in default is.)
Q2: The cheapest SP3 model comes with 4 GB RAM and all around other than the tiny monitor is a better system than those that were out when Civ V vanilla was first released. 4 GB might not be enough for a Huge map though, but with playing on Small I don't see that as an issue. (The more expensive models have 8 GB RAM)
Q3: Civ V will use a lot of battery so you'll want it pulled in if you intend to play more than an hour.
Also note that the Windows 8 default power settings turn down the power when on battery so you might actually experience slowness on battery that you shouldn't when charged in. (You can of course switch the settings to stop it from decreasing power usage, but then the battery will be depleted faster)
It should be possible when at home to plug a real monitor into it.
Steam allows updating from one computer to another, but I'm not sure that it's really expecting the same account to have multiple computers at the same time. This could present issues if you sometimes want to play Civ V on your current machine and other times play it on the SP3.
