Hiya!
I'm a first student studying PR, currently staying in Purbeck House.
To answer your questions, the rooms comes completely furnished and the kitchen (for Purbeck at least) comes with a kettle, microwave, iron and ironing board but no cutlery, plates, cooking utensils.
I'm pretty sure there isn't an option to pay extra for a double bed (unless you want a studio flat on your own, in which a double bed is usually a standard), but I think Dorchester has double beds - pretty sure all the other halls don't.
All of the main halls are extremely near to each other and the town centre - Cranborne, Purbeck, Chesil and Dorchester are all pretty much right next to each other, with easy access to the town, beach and ALL of the main clubs, so I'd go for one of them, as they're the most popular with the most sociable people.
Hope I helped!
- Hayley