I've done it. What time of year are you going? How many days do you want to spend on the mountain? I did the Machame route (most popluar) in January with a group of ten. Everyone made it to Uhuru, although one guy was puking pretty hard. We had the benefit of being military-types, and our command insisted we be provided Diamox. That probably helped some.
I can't think of the more sought after (i.e. expensive) guides, at the moment, however I can tell you that what you're largely paying for is food and some added luxury. The latter isn't a big deal. We ate on rocks at the rest stops vice folding tables. What I would have paid extra for was MORE FOOD, however. I'm 6'4" 230 with low body fat. That means I burn a lot of calories...especially whilst hauling my big ass up a mountain. I can vividly remember opening my tupperware lunch container to find a butter and cucumber sandwich, hard-boiled egg, carrot sticks and a cookie! After a few days I couldn't force down any more protein bars. So pack some Cliff Bars. They're tastier and a better choice, nutritionally.
Of course, you should be in decent shape, but altitude is what will get you. I was in pretty darn good shape, but that push to peak was still challenging. It's a struggle to just shuffle one foot in front of the other on a steep grade at 18,000'. Don't be too intimidated, though. My group combined two days due to time constraints. That meant stepping off from bottom of Barranco Wall at 0700, reaching our 15,300' camp at 1900, dinner and a quick nap, 2300 muster for 0000 summit push, then all the way back down to Mweka Camp (10,000'), arriving around 1600. So, we went almost straight through for about 33 hours. I wouldn't recommend it to others, but, like I said, we all made it.
That being said, you still have to respect the mountain. The reason we were required to take Diamox was because a doc in the previous group from our base got HAPE and almost died. He spent a week in a hospital in Nairobi afterwards. Just be honest with yourself if you think something's wrong and don't let your ego take control of your brain. Kili is an amazing adventure, but it's not worth dying for.
Sorry for the rambling. Hope this helps a little and let me know if you have any other questions.
Oh yeah, and don't pay more than about $250-300/carat for Tanzanite. The prices have plummeted over the last several years.