I didn't like Debarq: the atmosphere is sad, cloudy and humid. There's nothing to do; it's cold as it's at a very high altitude (2,200 metres), the buildings are ancient and the villagers live in great poverty.
As the park is expensive for the day, what with the whole team having to accompany you, you can spend a night in Debarq, before or after the trek, for the purpose of saving a bit of money, but the price-quality ratio of accommodation is not very interesting overall.
I'd advise you instead to organise your trek in the Simien Mountains from Gondar, which is what all the travel agencies suggest, so that you don't have to hang around there. You can negotiate an all-inclusive price (camping, meal, entrance fee, scouts, guides, etc.) at a better price than in Debarq.