As said, D5D is a very good choice for beginners or hobbyists. Many beginners will welcome the AS function as hand-holding is a challenge for many beginners. In addition, the in-camera processing is closer to PnS cams than many other brands which produces more "pleasing-to-the-eye" images.
D50 will provide a smoother transition to D70 or D200 if you get more serious with the hobby, as you would already have the compatible lenses, flash system and what nots.
Although the same route can be charted for D5D by upgrading to D7D, I personally see D200 (and D2X)offering (at this point before KM retaliates) more "professional" functions.
It really depends on what you are looking for in photography. We have also seen many who regretted moving to DSLR. as these people prefer the hassle-free conveniences offered by PnS, and didn't really need the DSLR to shoot what they usually shoot.
yeah km5D 18-70 3.5-5.6 DT lens. i think plastic mount but it's not a big problem. kit lenses are good enough. trust me. i shunned my 350D's kit lens and later ate my words when i bought one 2nd hand. good walkaround range.
the nikon 18-55 has an ED element. supposed to reduce CA and increase resolution etc etc, but in real world tests, you'll be hard pressed to find a difference.
if JSBN doens't pop into this thread to give his comments, PM him or see his post in the KM forum. you'll be amazed by the high ISO performance of the KM5D.
Nikon has huge range of lenses and accessories, original and third-party. They also have a huge user-base, which means it'll be a lot easier to buy/sell/trade your system, either in whole or in parts. And for some strange reason, whenever you hold a Nikon, laypeople tend to think you very pro. It's the brand name, lah. (I've heard of people comment, "That pro use Minolta only. Like that very good, meh?")
KM, in my (probably biased) opinion, is technologically more advanced than Nikon. Case-in-point, the anti-shake system. Once you use it, I swear you'll never want to use another system without it. Other brands may have VR lenses, but if I may jump the gun and make assumptions, the fact that you're considering a D50 probably means you won't be able to afford the VR lenses. And even if you do buy one, it's just one lens, and might not be the lens that you'd be using most of the time. KM's D5D effectively converts ALL your lenses into VR ones!