Why do those rifles always have to look so funky? I'd be out there with my Wal-Mart Marlin.
Yeah. You could do that. It'd be a way for you to start. (though I don't believe that the basic bolt action Marlin meets the weight requirement.)
The "funky" rifles in question are that way for a number of reasons (and remember that funkiness, like beauty, is in the eye of the beholder.)
The stocks allow for the gun to be custom fitted to the individual shooter's anatomy. The attachments allow it to be carried comfortably and conveniently accessible on a skier's back for many miles, mounted and dismounted with ease and speed. The sights - which alone will cost more than that Walmart Marlin - are extremely precise and durable and designed to protect the apertures from contamination by snow and ice . The action - the straight pull bolt - allows for rapid manipulation of the firing sequence without the danger of jamming that using a semi-auto would bring (remember the conditions under which these guns have to function). In addition to speed, the SPB allows the shooter to maintain position, nothing moves except the hand - keep his or her eye on the target and get the shots off faster (they are, after all, in a race).
That is at least a bit of why they look like they do. If an inexpensive firearm worked as well, competitors would be using it but they don't.
The Anschutz Biathlon rifle is shot mostly at 50 meters. At 100 - with the aperture sights, not a scope, from prone, not a bench - it will shoot one half inch ten shot groups with the right shooter and ammo.