Well...if you are unsure about mixing it yourself, and you're going to get it mastered anyway...then might as well give the mastering engineer a decent mix to work with.
Since you are going to get it mastered...if you have a mastering house in mind, why not ask them who they recommend for the mixing.
Not sure what you mean about spending money on promotion...like, what are you talking about money wise, and what type of promotion...but...if the mix isn't done well, then you're not going to want to promote it anyway, so I would say, get the mix right, and worry about promotion afterwards.
Oh...no, I've never paid anyone to mix, so I can't recommend anyone...but man, there are like a million studios that can do that for you...all depends on your budget. What is your budget for mixing and mastering of you don't mind saying...?