I need someone who will be effective (not dying) when the home is attacked - which is a lot. I am concerned that at high levels, my steward will be smoked.

Who is a good choice?

There's an option for your housecarl to become your steward of the new properties. Or you could marry one of the mercenaries to help protect your place as they get to be level 40-50.

Here's a list of candidates that have been confirmed to work or not work so far:
http://www.gamesas.com/topic/1409356-unofficial-list-of-stewards/

Personnally I might go with Ingjard of Dawnguard since she doesn't have a max level, that way she'll still be good at higher levels. Plus she's kind of cute

http://www.uesp.net/wiki/Skyrim:Followers

http://www.uesp.net/wiki/Skyrim:Ingjard