My son and I did the bit between Hexham and Bellingham on CX bikes. We live at that end and it was before it was officially opened. It was fine. There’s a thread on here somewhere with some pics if you search. I reckon an Orange 5 would be over the top unless you wear full body armour.
It is quite rough in places but there was nothing to really trouble a cx bike. If you are doing it over 4 days then it shouldn’t be too demanding in terms of miles so you might have more fun on the mountain bike. I felt a bit battered doing it on a cross bike. We thought it would be smoother (more like C2C) and wanted the speed of CX bikes to get it done in a day. Hindsight says my 29er HT with some fast tyres would have been better.