My question is: for tax purposes, can the house (and the value thereof) be considered an inheritance from my Dad or is it a gift from his wife?
Your facts were that his wife inherited the house under the will, and then she (not the estate) transferred to you the remainder interest in the house, while she reserved a life estate for herself, right? Under those facts, she made a gift to you of the remainder interest in the home.
If it is a gift from his wife to me, is there a limit on the value gifts from a non-blood relative (she is my step-mother) for tax paying purposes?
Under federal gift tax, if the value of the remainder interest she passed to you was over $13,000, then she must file a federal gift tax return. Unless she's used up her lifetime credit against federal estate and gift taxes (currentlly $5 million, but will return to $1 million at the beginning of next year unless Congress changes the law), then all that happens is she uses up some of her credit — no tax is actually paid until she uses up that credit.
As the person receiving the gift, you don't pay any gift tax so you don't have to worry about that.
To complicate matters further, I live and work overseas and claim the foreign earned income tax exclusion. Is the value of the house considered income of some sort?
Gifts are not income to you for income tax purposes, either.
You do need to figure out what your basis in the remainder interest is as you will need that to compute your gain when you later sell the property. She got the property with a stepped up basis in it because she inherited the property (I assume she was not a co-owner of it or she'd not have settled for a mere life estate.) So, an appraisal of the current value should suffice to establish her basis. You'll want that for documentation to back up the basis you use later—current appraisals are more convincing than one done years later when the IRS is challenging your return. Then that basis needs to be allocated between the life estate and the remainder interest. That's done proportionally based on the values of the two interests. For example, if the property were worth $100,000 and the life estate was worth $20,000 and the remainder interest was worth $80,000, then the basis of the remainder interest is $80,000 and the basis of the life estate interest is $20,000 (since, in this case, the basis of the property is equal to the value of the property). See IRS publication 551 on basis for more information on that.