Maybe it's just me being me (stubborn), but I still have my 'beginner' MTB, a 1995 GT Aggressor, that I've upgraded from the frame out.
Can't bear to part with such a sweet CrMO frame.
Had to add a Judy TT, Core headset and RaceFace downhills, plus upgrading the entire drivetrain to Shimano LX/XT, along with v-brakes and Mavic wheels. Can't upgrade brakes any further (sorry, no discs due to the frame), but I don't think there's anything left that needs attention.
I've gone through three pairs of SPD-compatible shoes, three helmets (just bought the fourth this weekend), three sets of tires. Only one chain, though.
After 15 years and countless beatings, several years of working the Front Range trails near Boulder and Golden before moving back to Florida, it's never let me down, so I don't mind spending more on it than it's obviously worth.
Even the original paint still looks good. Doesn't hurt that decals cover scratches very well.....
My recommendation; buy what you can afford, and upgrade as you improve.
It's likely you'll spend a bit more in the long haul, but at least you can be honest about earning those upgrades, right?