This recipe is great! I used it for a bride's taste test and I thought the groom was going to die it was so good, lol! http://cakecentral.com/recipes/17550/lemon-cake-starting-with-a-white-cake-mix
As for the icing, I just used my regular BC recipe and used a tsp of lemon extract in place of the vanilla and/or almond. HTH!!
Not sure about the cake but I ditto shelley on the bc. Just use lemon extract rather than vanilla, or whatever other extract you normally use. 
King Arthur Flour has a great Lemon Bliss Cake recipe on their web site. It includes a lemon glaze but I'm sure a lemony buttercream would go well with it too.
I just made this one last week....YUM!
http://cakecentral.com/recipes/7371/amazing-lemon-cake-from-a-mix
I just used reg buttercream.
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