New to Grizzly need advice. Thanks

Assuming by your mention of this being a winter evening project that you are planning to do the work yourself? Good. Can you go and pick it up yourself or will it need to be shipped? Has the Grizzly been sitting outside in the weather all this time or was it stored in a barn?

Doing the work yourself along with picking it up close by will be minimal cost. Yes, my opinion would be to go get it especially if you like working on projects and it isn't something rotting away out in the weather. That would be my opinion and I'm sure other's will have a differing opinion.

You can search around on places like sharkpart.com or partzilla.com to see what parts will cost you if you go the new OEM route. You could also try to source used parts for the parts you need. That bike is somewhere around a 1998-2001 model year although I have never owned one or worked on one. There are some on here with knowledge of the Grizzly 600 and are usually very helpful with trying to help diagnose issues.

Based on your limited explanation it would be hard for someone to pinpoint a price to fix the current issue. Don't forget an ICE machine having sit for a number of years will probably need the carb rebuilt, tires might be rotted, all fluids changed, stuff greased up, etc.

Will this bike be worth fixing up all depends on what's wrong with it. It sitting for 3-4 years did not good for it.

For the no drive issue. This could be any 1 or a number of things.
Could be stripped splines on the rear axle.
Could be broken yoke on the rear driveshaft.
Could be a broken belt.
Could be stripped splines on primary clutch.
Could be worn out wet clutch.

There is others that can added to that list.

Additionally to this, some parts are not available new anymore, and used parts for the 600 are getting hard to find.

Your going have to start tearing everything apart. Give everything a good inspection to diagnose what it will need, then make a informed decision on what to do.