Hello Riverman,
I've been using Nikon for a number of years, first film and then D100 and for the last year and a half, the D2X. I can't answer some of your questions about bodies but I'll try to give you my thoughts on the points I have experience with.
20mm AFD--I have had this lens for many years and continue to use it successfully on both digital cameras, although there are several things to keep in mind. On the plus side, the digital sensor on all Nikon cameras is less than full frame so it does not use the extreme edges and corners of the lens, the areas where any optical weaknesses are most likely. On the negative side, because of the smaller digital sensor the lens gives only the field of view of a 30mm on a film camera. For this reason, my most used wide angle lens now is the 12-24mm f/4 Nikon lens made for digital. If you like wide angle landscape photography, I think you should consider that lens as part of your plans (it is a really fine lens). (There is also a Tokina version of that lens that has had decent reviews and costs less, but I haven't used it.)
One other note about wide angle lenses on digital sensors (not unique to the 20mm or Nikon). Chromatic aberration is likely to be more easily noticed on a digital sensor than film and at least small amounts of CA are likely to be seen on any wide angle lens in the corner when viewed at high magnifications. The good news is that it is easily fixed in digital processing which improves overall image quality.
While I don't have either the D200 or D70, I feel comfortable in saying that there are measurable improvements in color quality from the D70 generation of cameras to the D200 generation.
If your goal is to print at sizes as large as 16 by 20, you will see a very definite improvement in image detail from the 6 mega-pixels of the D70 to 10MP on a D200.
The SB-28 works on my digital cameras (and I assume it would on the bodies that you mention, but I haven't verified that) but only in manual and auto modes, not TTL. To do TTL, you need the latest generation Speedlight such as SB-800. So, whether the SB-28 will do the job for you depends in part on how you use flash--if you work manually, there is no problem. If you want to rely on TTL, you'll need a new unit.
Hope this helps.