Seems interesting, but these guys are a dime a dozen now-a-days and honestly none of these guys will put a wholoe of time on your tracks. If you want that attention you will have to pay a lot more. That being said, the best way for you to get a solid product is to deliver a solid mix. I know it sounds obvious but ask any masteing engineer out there and tey will all tell you the same thing. The amounnt of work needed on the the master depends on the quality of the mix. Fact is, it is not uncommon to have good mastering engineers send back source material who's mix is to bad to master.
If this is your first master, I would really advise for you to be in the room with a pen and pad taking notes. I CANNOT STRESS THIS ENOUGH!!! For the first couple of sessions you WANT TO BE IN THE SAME ROOM!
Once you feel comfortable with the gain stage and mix levels that you and your mastering engineering agree on, it will be very easy for you to just pump out mixes and sound them to the cloud to be mastered (with stellar results).
Initially though, you want to take it slow absorbing as much information as you can as it will be saving you tons of money in the future