Hi bud,
I regularily drive 300 miles plus so no issues with comfort. As this drive is to London I often crawl in heavy traffic, no problems here which are car specific.
Regular motorway driving should be 30+ mpg. In heavy traffic you will be in second and third a lot so probably about 20+ if you take it easy.
Servicing in warranty at Nissan for convenience. Cost very much depends on where you are in the country. Dealer service can be very hit and miss, some are ok some are awful. Do not rely on anything a dealer tells you, they come out with some crackers.
Would seriously suggest using an independant for servicing. We have some good guys on here but again depends on where you are.
Buying from a dealer may be good in terms of warranty but I would be equally interested in the cars history. Buying from a forum member would suggest that the car has been very well looked after and in many cases better than when bought.
The insurance lottery has always been beyond me but the £850 sound pretty good.