Is a gary fisher 05 marlin a good beginner bike?

If it is in good working order, and you don't try anything too crazy with it, it should be an ok bike to start with. It might not be quite as capable as a modern bike, but you can still ride it like a mountain bike.

Probably really only need one thread for the bike but in the end it is a bike, nothing exceptional but potentially rideable. My guess is the end the fork likely hasn't been serviced, new rubber is needed, new chain, cassette and maybe cranks (for the chainrings as chainrings themselves are sometimes more expensive then new cranks). The bike will likely also need a tune up and maybe some fresh bearings. In the end it probably won't be worth it but see what your local shop says. A new bike of quality for mountain biking is probably going to be 1200+ and that should come with an air fork and ideally 10 speed or higher.

An old entry level mountain bike could be ok for certain stuff but if you are new to it you might want something with better components, more travel and wider tires. It will certainly make it easier. However plenty of people have ridden old 26" mountain bikes and survived so it is up to you in the end. We haven't seen the bike and don't know condition or your abilities so it is a bit of guessing on our part.

There's no reason why it shouldn't function adequately unless the fork is "sacked out". I'm still riding my 2005 Santa Cruz even after 25K -30K off road miles.

That generation of aluminum Gary Fishers were known for cracking. Check the welds and ride it until it fails.

Were those made in Taiwan ? And what grade of aluminium tubing quality was used ?