For a reliable, quiet compact car with a luxurious interior and comfortable front seats check out the Buick Verano. Economical--sort of. The Verano's EPA ratings of 21 city, 32 highway are towards the bottom of the range for sedans its size. The Verano's other weakness: a tight back seat.
You might also try to find a Lexus HS 250h. A hybrid sedan based off of a European-market Toyota, it's a good fit for your priorities.
Either is available well under your age / mileage limits for under $12k.
Other possibilities include the Acura ILX and 2014+ Kia Forte, though these aren't as quiet and smooth as the Buick or the Lexus. The Acura also has a tight back seat.