The front suspension only needs about 2 inches of travel, just enough to soak up the sharp blows. It shouldn't weigh more than 2 pounds.
Currently, the shortest forks I'm aware of are 80mm, about 3". They are much more reliable and deliver longer service intervals than the 2" forks of 10 years ago. They're also stiffer and lighter. They are superior in every way. I doubt you'd find a suspension fork at 2lbs since rigid mtn bike forks barely squeak in below that (most are above 2lbs). My 97 vintage Manitou SX Ti has 77mm of travel (just under 3" and weighs about 3.5lbs. IMO, that's as close as you'll get to your ideal fork. Current renditions are available, but are not cheap.
No rear suspension, but the frame must be able to soften up the bumps.
I've ridden a lot of bikes, but I've never ridden one (mine or anyone else's) that had that magical quality. Sure, mag writers talk about that a lot, but it's a pipe dream. Kinda sounds like a gun rag writer, eh? Better to get a good stiff frame and put fatter tires on it to absorb the blows. If there's anything good to come out of the freeride/DH/dirt jumping fad is superfat tires.
Rim brakes are good enough, while being sturdier and lighter than disk brakes.
An finickier to adjust. It's nearly a black art to getting rim brakes properly adjusted so they don't squeal. I'll take the weight penalty of a good set of disk brakes.
9 or 10 speed rear cassettes and shifters are the wrong idea - someone should design a 5 speed system that's smaller, lighter, and more reliable.
Current 9spd systems are better shifting and just as reliable as the older 5/6/7spd systems. One reason is that the chain makes smaller jumps at each shift. Actually, with the advent of cassette hubs in the late 80s, we hit a pinnacle of strength, low weight, and reliability not seen with the previous hub designs. I've ridden everything from a friction shifting 5spd hub to fully indexed 9spd systems. I'll take the latter please. There's no significant weight penalty with the additional cogs.
The rest of the components should be high quality and light weight. The whole bike should be built to last 10 years of serious riding, it isn't the kind of bike that gets replaced every season.
That bike is available now if you can be flexible with your design parameters. My bike is pretty close to what you ask for even if it has more travel in the fork and a 9spd cluster. The only maintenance I perform is to lube the chain and adjust the brakes. I haven't had to touch the drivetrain or various bearings except to replace one cartridge bearing in the rear hub. In two years, I raced that bike for a season, rode it in a century (100miles in a day), and 2 MS150s. I also averaged close to 100miles a week in various types of road training and recreational mtn riding. It sat nearly idle for the past 5 years, but when I pulled it out recently, I didn't have to do anything to ride it. It went straight from retirement to trail. I didn't buy that bike though, I built it. That's what you'll have to do if you want your dream bike.
Chris
