I have an old ’81 Honda CM400. It’s been a good bike for me. I’ve put almost 4,000 miles on it without having to fix anything other than putting a new headlight in it. I guess that’s pretty good considering the bike is almost 30 years old. As for power its got plenty of power for solo riding, two up it is taxed a bit however it’s not really big enough to ride two people on it for long especially with me at 230lbs. I live in a pretty hilly area nothing huge though. Almost all of the hills are really curvy as well so it always ends up being the curves limiting the speed not the power of the bike. The one hill that is straight I have no problem maintaining the speed limit of 55 however I do have to shift down to 4th to go that speed. Funny thing is if I run it up to about 75 it will pull the hill at that speed in 5th, just the way the powerband (or lack there of) is.
That being said if you are looking at an older 400cc bike for the gas mileage I honestly think you’ll do better with something like the Vulcan 500. If I baby mine around I can eek out about 57mpg. Usually I am in the 52-53mpg area. Most people I have read about with the Vulcan are getting 55mpg or so and it has a lot more power than my 400 as well as being a bigger more comfortable bike. If you are looking for mileage that is significantly better than that then your only choice is going to be one of the 250’s.
I personally have no experience with 250cc bikes other than riding on in the MSF course. One of my friends does have a Suzuki GZ250 but he says its pushing it pretty hard to maintain the speed limit on the hills around here. I think that the GZ is the least powerful 250 though and the Honda Rebel and Yamaha Virago have a little bit more power.