A 2000 Monty 315R would be a great bike to get in to trials on, I might be bias though ;)
Parts are available though "local" dealers but they typically are not on every corner. Check out
www.usmontesa.com for locations of dealers. Parts are generally available and if you keep Lewis Sport on your list you should never have an issue with parts. They are great people and VERY dedicated to the sport! The Tryals Shop
www.tryalsshop.com is another great resource, Mike Komer is a great guy and knows trials! Ryan Young typically has general parts and a great video at
www.rypusa.com
Get a shop manual from a dealer or maybe from an auction site. They are VERY detailed and should cover anything you might ever need to do to the bike and then some!
The Monty engine is VERY well built, duh it's HRC. I do think at least ring would be a good idea on a bike that age though.
The 80:1 premix is no problem just run a VERY good quality synthetic. I both my 315 used from a high level rider who ran it and I ran it using Amsoil premix and have since sold the bike to Rich and I know he's been putting time on it as well so it doesn't seem to cause any durability issues. It's acceptable because the bikes are not run at high RPM for extended periods of time idle to off idle are the majority of time on most trials bikes.
I used VP MR2 at 100% and had no issues. Rich suggested it to me and I always have very crisp throttle response. Since they use such a small amount of fuel anyway using the good stuff is not that expensive.
There should be an extended range tank available for the 2T Monty as well, call Adrian or Mandy at Lewis Sport and ask.
Congrats!