Do I even consider this mare?!

A horse I am interested in buying has front fetlocks VERY slightly turned in (you wouldn't notice unless it was pointed out). I was wondering how much this would affect her riding capability?

Honestly? I wouldn't worry about it, unless it's worse on one side than the other, or a moderate to severe fault. You say it's very minor.

I have found that toes slightly in in the fronts and toes slightly out in the hinds is very normal and can actually be a good thing, makes them more sure-footed.

Yeah... I think I might not get her just to be on the safe side.

Your decision :) I had a gelding that was toed-in in his fronts and he was awesome. Best jumping pony. Totally 1000000% sound, jumped 3' (and he was 12.1) and walk/trot/canter on hard-based gravel without missing a step. I only know that because he decided I'd let him get too far behind the other horse we were riding with, and he bolted to catch up - and I couldn't stop him for the gravel.