Anytime you get a second hand instrument, it's a good thing to change strings. One at a time -- do NOT take them all off them put them all back. If the bridge or nut aren't in exactly the right place it can really mess things up if one or both fall out of place, and string tension will keep them where they belong.
What is the VSL? That's the distance between the inside edge of the nut and the inside edge of the bridge. THAT, plus what you want for the home tuning, is what tells you which strings to buy. There are charts and calculators, but virtually ALL pre-packaged sets of "dulcimer"