List of essential tools to maintain an Ortler Mainau bicycle
Lennard Zinn has written books on maintaining road and mountain bikes (a trekking bike probably falls under the "road bike" category in terms of maintenance). He lists four levels of tool kits based on how ambitious you want to get with servicing your bike. I think this is a reasonable approach, especially since we don't know how ambitious you want to get servicing your bikes in particular.

Also, to some extent, a tool's necessity will be determined by what kind of bike you have, for instance, my road bike has no torx bolts; with a carbon bike, a torque wrench is probably more important. So any advice also needs to be tempered by the bike you're servicing.