My Yamaha FG260 12-string is now 30 years old, and I've never had any trouble keeping it in 'concert'.
I worked on it when it was new (the day I bought it!) - a bone nut, re-tensioned truss rod, and I planed some wood off the top of the bridge so I could lower the bridge insert. A bit drastic, but it worked really well.
These days I use Kaman 1717 strings, and it really sings.
Phil (Mudcat name FOG - Friend of Gnome) has a similar instrument which he bought secondhand recently, and it's a shed in comparison... but it will be fine with a bit of work!!