Well, this depends on where you end up. In concourse A, Schengen bus arrival currently requires you to take the stairs up to the departure level and then once again down into baggage claim (in both cases, there are elevators you may use instead). Non-Schengen A bus arrivals will probably drop you off in concourse B.

I have never arrived in concourse B (both Schengen and non-Schengen), but in concourse C (sometimes a concourse B bus arrival will drop you off at the concourse C arrival gate) you won't have to deal with stairs as you'll already arrive on the customs and baggage reclaim level. But it may be a bit of a walk, especially in concourse C.

Absolutely nothing to worry about, and DUS is quite accessible (although you may have to do some additional walking if you have to use one of the quite sparsely spread elevators especially in the main hall.

Additionally, there are lots of finger gates, so most bus arrivals will actually be shorter hops within the Schengen zone, although evening arrivals on BA from LHR are quite likely to get an apron position.