Thesis Committee Help!

I'm at the point where I've started putting together my thesis proposal and forming a thesis committee. Technically speaking, this is required to be complete within one year of completing comps. One year would be the second week of January... but while this seems like I'm waiting until the last minute, I'm actually the first student of my year to actually start to formally put together my committee (let's just say that the program let's this year deadline slip unnoticed). My background is complete, I have my big-picture general hypotheses written, and three main aims with multiple sub-aims. My PI/adviser said today that I have roughly 140% of a thesis on paper, meaning that I have more theoretical aims than I will need to complete, but we both think this is a positive since it's easier to cut out part of a project than add to it.
At this university, thesis committees consist of 5 individuals: the adviser, 3 other internal members (one of whom must serve on the program oversight committee), and one outside member. I know who my ideal internal members would be, but my adviser is the only one on the oversight committee and he doesn't count since he is my adviser. Unfortunately, this means that I have to eliminate a potentially very useful committee member to make room for another potentially less-useful member. As far as my outside member, I'm going to wait and see what direction my experiments take over the next year before I decide if I am pursuing a crystallography direction and thus would want a crystallographer, or if I am pursuing a helicase direction and thus would want an enzymologist. My PI had great suggestions for one of each, both within driving distance, so I am optimistic for finding a suitable outside committee member.
That said, I now need to formally ask the internal members to serve on my committee. Female Science Professor's recent post on seeking advisors has made me think about the best way to ask these individuals. After all, there is really little benefit for agreeing to serve on someone's thesis committee -- published papers will go under my PI's lab, not theirs, it's unlikely that (with the exception of one of my hopeful committee members) I will be doing major collaborations with them, and it's a lot of time and effort on their behalf. To add to that, my project is way out in left field in comparison with everyone else in my program. Almost all the Ph.D. students are studying cell signaling, mainly in breast or prostate cancer. As you can see from my above comments on crystallization or enzymology, my project has nothing in common with signaling or cell/molecular biology in general -- I may as well transfer to the biochemistry department at this point. Additionally, my project is really trying something new and unique that applies to the entire field of studying disordered proteins, much more so than just studying the particular forms of pediatric cancer studied by my lab. That said, I am super excited about the project, because it has the potential to wind up being something completely novel within biochemistry and widely applicable to the field of disordered proteins as a whole. I want to express this interest and excitement to the potential thesis committee members as a way to encourage them to agree to serve on my committee; while it may be more work on their behalf since this isn't a topic they see everyday, scientists should be excited by the new and unexpected, right? Especially when the new and unexpected is coming from a graduate student!
I asked a few older students how they formed a thesis committee, and most said they just sent an email basically saying "Will you serve on my committee?" My plan at the moment is to send a brief but well-written email stating why I think they would be a good fit for my committee (i.e., their involvement with biochemistry, their tie to the small molecular inhibitor side of my project, etc,), how much I value their opinions (particularly from those from whom I have already seeked advise), and attach a brief summary of my thesis proposal -- three paragraphs on background, my hypotheses, and the general aims and sub-aims, but without the proposed experiments. It totals 2 pages, and a significant chunk is in outline format. I don't want to send too much to read, but I want to show that I have put a lot of thought into the project thus far, and I am not blindly asking them to serve on a committee without a strong idea of the direction in which I want to take my research.
For those who read this blog who are faculty members, I'm hoping you can provide me with some suggestions as to the best way to ask these committee members. What impresses or disappoints you in regards to being asked to serve on a committee? I know it is a LOT of work for faculty members, and I really want to assemble the best possible thesis committee that I can.