1. Can I do my own maintenance on this model;

Yes you can. Though there's less space to work in the 850 engine bay due to the tranverse mounted engine.

2. Will the 850 be as dependable as my 240;

I own both. The 240 being simpler in design is the more dependable.

On the other hand, the 850 is a modern 'drive' and handles miles better than the 240. It's also more comfortable in my opinion with much more supportive seats and a better cockpit. Space wise, it's wider than the 240 wagon.

3. Do you recommend this model or is there a better one? I do need a wagon to carry my dogs (Great Danes), and I drive over 150 miles a day in Northern Maine, so I will need the AWD.

If you need AWD, the 850, and V70 are the only models with AWD variants in the years I think you're looking at.