Generally speaking, going from a fixed sight to an adjustable usually requires some modification to the frame (milling) so that adjustments can be made to the sight. That depends on you some as well. Many prefer as low a profile as possible (milling necessary to achieve this). Some fitting will be required either way. Plus, you have to consider that you will most likely need a new front sight post as well. The best adjustable rear I have ever seen or used is the Bomar. When properly mounted, it is second to none and is still the top choice among bullseye shooters.
The information OldColt gave you is mostly correct. Though for sight changes I usually have the slide milled, not the frame. (
I could not resist oldcolt
Concerning adjustable sights it really depends on what type of sights you want and which pistol you buy. Since you mentioned buying a Kimber Classic Custom with the fixed sights and you might want to change, do you want target sights or do you want adjustable combat sights?
If you want adjustable combat sights there are a couple of options. The best sight that fits this description is made by Miniture Machine Corporation, MMC. MMC does offer sights that will fit the standard Kimber dovetail in the slide so minimal fitting should required. There may be other sights available, from Heine, or Millet that will fit the bill nicely. I have a set of MMC sights and I do like them. Though since these are combat sights windage is a drift adjustment just like the standard sights, and vertical adjustment is rather coarse.
If you want target sights, oldcolt is correct. Bomar's are the best. He is also correct in that you will need to have the slide machined in order to fit the sights on your slide. On the other hand it may be to your benefit to buy a Classic Target to get the factory installed adjustable sights. There have been reports of these sights breaking, and Kimber replacing them under warranty. But Bomar does make a replacement sight that uses the Kimber slide cuts, so fitting should be minimal. Also, Heinie is reported to be making fixed sights that fit the slide cuts for the Kimber adjustable sight. So you do have some fairly easy options.
I hope this helps.
The best handgun for self defense, is the one you have with you.
[This message has been edited by Str8_Shot (edited 11-18-2001).]