Had the vII of the 90/2.0 for my M3&4, a big and heavy lens but wonderful optics. After getting the M9, picked up a 90/2.0 vIII(E55,pre asph), then started looking at the Elmarit-M(90/2.
with the idea of going smaller and lighter. With the ability to bump the ISO on digitals, raw lens speed is not quite as important as with film, especially with a lens that's not used frequently.
Unfortunately, Leica's naming conventions are a bit confusing. The best on-line guide is:
http://www.antiquecameras.net/leicamlenses.html.
One good thing about picking up good deals on used Leica stuff is you will probably recoup your investment when it's time to sell, if not make a bit of profit. I finally picked up a 90/2.8 vII, so the Cron will go. I had also considered the new Summarit(2.5), which had generated very favorable reviews. As I like the contrast of the legacy lenses, I went with the older Elmarit. I now have a four lens kit that can be comfortably carried in pockets or a fishing vest.
There are several lens/hood configurations that will partially block the viewfinder. And you-all do use hoods, right? After awhile you get used to it. As the rangefinder makes for easy two eyed operation, it's a simple matter to use the left eye to check what may be happening in the blocked portion.
The latest version of the 90 2.8 Elmarit-M is the type II(E46-filter size). Good review here:
http://www.stevehuffphoto.com/2010/06/22/the-leica-90-elmarit-m-f2-8-quick-lens-review/. Dont confuse it with the 90 Tele-Elamtit M, which seems to have a spotty reputation and should probably be avoided.
Any non-coded lense will work fine, and most can have six bit coding added later.