1) good accommodation- preferably en-suite
What do you mean by good? For example: Room A is a large room with seventeenth panelling and a great view of beautiful buildings and/or gardens. The nearest bathroom is two floors below the room. Room B is a small room in a modern block with a view of a modern block, but it has an ensuite bathroom.
Size, age, and quality of accommodation varies within colleges and from college to college.
2) medium-sized
Are you measuring size by extent of buildings and gardens or by number of undergraduates, graduate students, Fellows etc?
3) doesn’t need to be really wealthy but not poor either
Wealth is relative. None of the colleges is skint. Some are loaded.
4) good for law
Every college that offers law is good for law.
5) not far away from the centre
No college is far from the centre. Oxford isn't a big city.
6) diverse in terms of state vs private school and race
Every college is diverse.
Oxford is not booking.com or expedia. I suggest that you throw away your checklist, and apply to a college which you like the look and feel of. Bear in mind that, if you receive an offer of a place, the offer might come from another college.
Reliable sources? Look at the websites and/or social media of the university and the colleges. They don't need to advertise, so they don't gild lilies.
PS: the plural of criterion is criteria.