Moots ybb questions

For starters, looks like 26" - if so, tire choice will be limited for gravel. If you're a taller rider as the picture seems to suggest, 700c/29 would be better for you.

That looks like a good start. Roughly a 2000 Moots YBB? You could do some minimal upgrades and ride the heck out of that thing on some gravel.

There's still some good 26" tires being made, so getting nice tires and then making sure the bike is comfy (seat and grips) is going to go a long way. Specialized does a buy one tire get the second half off sale in March or April every year. A 26" x 2.1" Renegade or Fast Trak would fit that bike and be good choice for gravel. They appear to be 20% off right now which isn't much different than essentially 25% off in the sale mentioned above. People will recommend going tubeless, but I have no idea how realistic that is with the current wheelset. Rim brake, tubeless, 26" wheelsets are out there, but not common. Here's one on ebay at the moment.

If that's a mountain bike with three chainrings up front, it probably has all of the gearing you could ever need. Some new shift cables and housing could make everything shift a bit better if they haven't been updated in several years. Same with the brakes.

Sweet older Moots!

I'd consider going to 650B, all depends on what will fit tire wise and if you are comfortable with the added BB height. Both the brakes below have the adjustment you need:

Paul Motolite 
https://www.paulcomp.com/shop/components/brakes/linear-pull-brakes/motolite/

Box Two V-Brake 108mm (long version)
https://www.boxcomponents.com/Box-Two-Brake

Or you might be able to use these:
https://www.boxcomponents.com/Box-Two-Brake-Post-Extenders

I have converted 26ers to both 700C & 650Bs lots, so send me a PM if you have any questions and I will try and help