You haven’t said where you are travelling from and that might influence your choice as getting to the likes of the far North West can be quite time-consuming.
If you can take your holiday outwith the school holiday periods (remembering that Scottish and English schools holiday are not the same) then you can reduce your accommodation budget.
As above – there’s not a massive amount on Arran.
As well as the Highlands, consider Dumfries and Galloway. There are some excellent trail centres, some really good “natural” riding, beaches, lochs and a full choice of accommodation – including caravan parks.
The suggestion of Aviemore has already been made. It has also tons of good natural trails and easy access to some good trails centres and accommodation aplenty.
If you can narrow down your search a bit then you will get loads of advice on here about where to stay and where to ride.