I have done business with and recommend the user Shakaru. He has done many mining contracts with users here (though I'm not sure if he has done one quite this large). Another option is the user Vladimir, which is another respected member of the community.
I'm not sure anybody currently doing contracts can satisfy your item#5 - not only is unreasonable (nobody keeps an extra 100 GHash on the back-burner in case a client wants it - they would make more money using that 100 GHash themselves), but its against how contracts work (if you contract X GHash, then that is your contract - things get too involved when dealing with such individual stipulations).
Regarding item #2, "low priced" probably varies a lot from any individuals perspective. It will never be "cheaper" to mine on contract VS. owning your own equipment (those offering the contract have to make something in return to make money and account for depreciation of their hardware ). Like anything else, there is a balance. Going with a smaller contracting company (or individual) will likely get you the best price, at the cost of some increased risk. Going with a larger contracting company won't be as cost-effective, but your risk is lower. This is probably the big picture you need to look at and see where you fall! (and in this instance, Shakaru and Vladimir fit this analogy perfectly - Vladimir is the professional, and Shakaru is is more of a "mom and pop" shop). Personally, i would trust doing business with either. I'm sure there are others offering contracts, but at this point you need to weigh your risk vs contract price - everyone has their own balance.