I will risk sounding completely bonkers here, but I would give it a house blessing ceremony. If you are of a faith, you can ask a local cleric to do it for a small fee or donation. If not, have your own dippy, hippy version. This is to exorcise the horrible memories of the state they left the house in and to make it yours. Some people burn sage (it makes me nauseous) but you could light loads of candles, bring lots of fresh flowers into the house, burn incense if you like it, and walk from room to room, opening windows to let out the old, and placing candles and fresh flowers to bring in the new.
You can also create little corners or altars to things you love doing that are focal points in each area. So if you love reading, create a cosy place to curl up with a great chair and soft throw, cushions, a side table with a lamp and small pile of good books. If you are all sporty, create a neat organised area in your hallway where sports equipment is displayed, ready to be used. Etc.
Bit by bit, reclaim the house. Focus a lot on the front door too so that it invites you in and really feels yours with outdoor lighting and planting that welcomes you and isn't a reminder in any way of the previous owners.
Think of the house like a rescue pet,. Poor sod having been neglected by the previous owners.
And on a practical note, have a word with the solicitors. People who don't clear their stuff could be in breach of contract and fined for any expense incurred by you (skip hire etc.)