I'd say all the German HSF wrecks are interesting, the cruisers especially as they are on their sides rather than upside down like the battleships.
The block ships are not particularly interesting as wrecks but are very scenic and the Tabarka is unusual as you spend virtually your entire dive inside it.
For accomodation there are two options, liveaboard, where your boat docks each evening in Stromness and you bunk on the boat, or live ashore where you stay in Bed and Breakfast accomodation and walk along to your boat each morning. Liveaboard usually has the option of having breakfast at extra cost. Liveaboard with breakfast, air, weights and cylinders will cost around 200-250gbp per person. Live ashore will be a little more expensive. All the boats offer nitrox as an extra cost option and some also have trimix.
I've experienced a few boats up there and they can all be recommended. They are the Jean Elaine, the Halton and the Sunrise. However, I've never heard anyone complain about any of the other boats. The Orkney Dive Boat Operators Association (ODBOA) will probably be able to tell you more. One thing about the boats - do not expect luxury on any of them - there will be no mints on your pillow!
Don't forget to factor in the cost of getting to Orkney, flying there is expensive, I could fly to the US and back for less money. If you're not flying up it will be a long drive - about 12-14 hours from central/southern England. You will also need to budget for your ferry fare, which is around 30gbp for a foot pax. The ferry operator, Northlink, will provide containers for your party's diving gear free of charge, but you have to pre-book.
HTH