I'm no expert but I'll try to answer your questions IMHO as I have LT150:
1. I understand that the only differences between the 150 and 150z are the light output and zoom lens. Is this true? Since I am going to have a fixed setup for this unit what is everyone's opinion on the necessity of the zoom?
Yes, even the bulb is the same. The LT150z upped the voltage slightly from 3V (LT150) to 5V (LT150z) to give more lumens. The economode of LT150z is rated at 700 lumens and 1500 hours using the same bulb which suggests that our LT150 probably will fare OK at 1500 hrs too. I personally will push mine passed 1000 hr though I'm nervous about it

The zoom is nice but not necessary as you of all should be an expert at positioning since you had CRT. Basically you just ceiling mount the LT150, shoot it at the wall and buy or build a screen and position it to match making sure there is no keystoning or focusing problems of image. Some build a zoom by putting LT150 on drawers on glides to slide to and fro. This may be important with 4:3 images on 16:9 screen to shrink image to fit into 16:9 screen. You can also shrink image using "native" resolution mode on LT150 or use software with HTPC (YxY or Zoomplayer).
2. I am hearing the 1080i is a problem for the unit. I have a DTC100. Is this going to be a problem for me?
Most users report perfect compatibility at 720p for DTC100 and HDTV is beautiful. I would not worry. Some were successful at 1080i with some modifications of the cable (removing pins) or other voodoo moves
3. I would imagine that there are many of you who have a scaled DVD input as well as a HDTV input. Do you have an RGB switcher or how are you getting around the lack of 2 RGB inputs?
Kramer manual switcher is $130-$200 with good quality. Your external scaler may do switching too.
4. I will be ceiling mounting this projector. Does NEC or anyone else make a mount for this? How is everyone else mounting the projector?
Peerless make one. Others make their own. Search here for details.
5. Here come the big question. I have seen a few other DLP projectors but only in HT stores. Namely the Sharp and the Seleco (I beleive the SVGA version). I know the the Sharp has the RGBRGB wheel. How about the Seleco? I did not seem to be sensitive to "rainbows" from that projector.
The RGBRGB or 4x wheel is less rainbows than LT150. You should check one out in person or find a company with liberal return policy in case you can't stand rainbows. Most charge 15-20% restocking fee. There is also demo program or rental program you can use to check out PJ. If you let us know where you are, local members may audition their units for you to see.
6. For those of you who have seen the Seleco what was your opinion of it? For me it was OK, but not at that price point.
I have not seen it but for that price I don't want it. I'd rather buy the Sharp z9000 for 7K.