Upgrading everything or buy a new bike?
 

I have an older (1996 model) Diamondback Zetec mountain bike. I rode it for 8-10 years for a total of around 18,000+ miles total. All of the components are completely thrashed. Don't know what it would now be compatible with, if anything at all. It came out before the advent of the V-brake system. I love my bike but have been told I would have to replace pretty much everything except the frame, seatpost, seat, and handlebar. Is that too much to do for a bike I grew to love so much? Thoughts, suggestions, input?

Are you sure you would need to replace that much stuff? A usual suggestion on here is to ride until broken then replace because most things fail at different times. If frame, post, bar, and saddle are all you can keep, then new bike. Though I find it highly suspect, assuming that the saddle is original and you took relatively good care of your bike, that so many parts would outwear your saddle.

Well, last time I asked a shop, they said I'd need shifters, front and rear deraileurs, rear casette, brake system. I think they said I'd need a new bb 'cause they'd already cleaned it up as much as they could and just couldn't tune it up any more. I may have a go with another shop. I'm not mechanically inclined so it's not feasible for me to do the work myself.

Beside the price of the parts, labour hours are very expensive. If you can not refurbish the bike yourself, buying a brand new one (in the same quality range) will be cheaper than having it fixed.

That nails it.

One can inspect and replace only needed stuff BUT if that's not u and u have to rely on a third party, then buy a new bike.

Yup. Unless you already have the tools and stuff, it is not worth it.