i guess it depends.....
Everything is good with respect to each manufacturers but it all depends on what you think is good so i suggest make a list of what you think looks good (regardless of what anyone says) and test ride them. (600's, 900's, 1000's, 1800's etc..) just because they look good doesn't mean you yourself will be comfortable on it :)
Personally, I have tried so many that i will no longer buy anything lower than a 1000cc... (it just doesn't sound or feel right reving a 600...)
Don't be fooled by the sound of any bikes as any bike with a proper muffler regardless of power sound excellent! I was blown away with the sound of a 250cc with a properly tuned muffler setup.... Very Nice!
At the moment, I have 2 Honda Blackbirds, and a Kawasaki. A Hayabusa 08 coming soon.... :)
Tax in the UK... i can't help you there.
For millage, the lower is always better (in some respect) but do keep an eye for owners (sellers) who actually takes very good care of their bikes. There are riders who are passionate with their bikes and there are riders....
Kawasaki ZX-14 / Honda Blackbird / Suzuki Hayabusa / BMW K1200s with 15000 miles is pretty good in my opinion. Its basically young for these types of big bikes. Anything below 1000cc, well 15000 miles may seem a lot for me. A lot of riders do ride 600cc pretty hard to get the power satisfaction; on bigger bikes, well, its effortless :) (again personal opinion)
I have seen and ridden bikes that have more than 60000 miles on them (Honda & BMW's) and these bikes have been taken care of properly. (Serviced properly, small problems are changed not just mended, etc.. etc..).Very Smooth and a pleasurable ride! but then again always check service booklet.
I would recomend buying new (as i always would) only if it fits in your budget. otherwise the above is what i would be looking at including looking for scratches, dents, see if it has been dropped.... Was it fixed.. etc.. etc....
good Luck and happy riding.
Oh yeah, I personally don't worry about the mpg... if you will look into this, stick with the 600's.