I looked at these some time back and was sorely tempted by one. I made up my mind to get one then was faced with the fact that they had ceased production. Now they look to be back in the Catalogue so should be available.
Not sure what your skill level is but they quote ‘this model is for expert builders’ in the instructions assuming you fit that bill then I reckon it will meet your requirements. However bearing their words I would suggest that you do indeed download and read their instructions, you would then be able to judge for yourself your competency to build this fine kit.
As for getting an unpowered one I would imagine that the suppliers would be happy to discuss this option though I can see nothing in the instructions that indicates if both bogies are powered, I would be tempted to get 2 kits and double power the one using the second as a trailer. The bogies look to fit in a similar space envelope, though you will be looking at £800 for this option. Will very much depend on the discussions you have with them and your layouts gradients as to whether a single powered Railbus will cope with a trailer attached.
Finally if you want the yellow Whiskers for the ends, they are a payable extra.