Towing?
 

What is the best vehicle to tow a 2000 pound trailer based on mpg and price (used)? Looking to get about 40 mpg and under $7k. Transmission and body style are not factors. Thanks!

turbo-diesel with a manual trans.

A VW Golf TDI should be rated to pull that much, most hybrids are not rated for towing but most compact cars are.
If you are really worried about using a smaller vehicle and are going to be towing the same trailer every time, look in to getting trailer brakes for the added safety of being able to stop quickly.

An open or enclosed trailer? And what purpose?

The shape of an enclosed trailer has more to do with how well it can be pulled. This also is important for resistance to natural and man-made winds (think big truck bow wave). Longer -- but lower in height -- should take precedence over tall but shorter in length. Narrow over wide as well.

Highly radiused (rounded) edges are fundamental to an "aero" trailer . . the horsepower demand is greatly reduced.

The closer the height and width to the tow vehicle, the better, as an overall rule-of-thumb.

The proposed trailer weight is so light that nearly any vehicle could pull it, given a good analysis.

Pics of the trailer, if available, would be a help.

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As for using a Golf to tow a 2000 lb trailer, have a look here. You'll find a dozen or so conflicting opinions. After reading it all, my take on it is that it would be a stretch to tow 2000 lb with a Golf.
VWVortex.com - VW Golf towing capacity??

Is there a diesel powered pickup that's not huge? Maybe a six cylinder?

DOT and the SAE say that compact cars are legal to tow 2,000 pounds, mid size cars 3,500 pounds.
I've towed around 2,000 pounds with my Civic VX and it did fine, if I was on a racing forum or other high performance forum then my opinion might be different, but towing a trailer should not be a race.

The trailer would be a lightweight teardrop trailer that expands, like the www .safaricondo.com/ alto1713/ indexeng.php, but a bit smaller. The vehicle is meant to pull the trailer 90% of the time.

You are not going to find a pickup truck with an EPA rating that is over 27mpg that was built in the last 25 years.
I know a hand full of people with VW golf's that tow trailers with them and are happy with how they tow, partly because the golf is a heavy car for it's size, weighs about the same as a toyota pickup truck from the 1990's, so you don't have to worry about it being tossed around in side wind as much as might happen with a lighter car, a hatch back is also going to have less distance between the hitch and the rear wheels so it will be more stable and less prone to fish tailing.

I've towed well in excess of "recommended" limits with every car I've owned. The "safety" is based on a general consensus that doesn't necessarily account for driver skill, attitude, or real world conditions.

Every part of your vehicle has an actual "safe" limit, and that's literally the limit at which it may fail, potentially catastrophically. Anything below that is technically as safe as the person operating it.

Id like to add that, anything can pull anything, but not everything can stop anything. Once saw a ranger pulling a 32ft flat trailer of round hay bales.... id sure hate to be in front of that.