I think www.edmunds.com 's used car appraising site is
helpful. It breaks down each year and model's appraisal by
mileage, geographic location, condition, and whether one is
hoping to buy from a dealer, private party, or via trade-in.
It also has city/highway fuel mileage stats for a given car.

www.fueleconomy.gov has fuel mileage stats also.

Try this site
http://www.fueleconomy.gov/feg/findacar.htm

Consider running a carfax report. Try carfax.com
or do a google search for Car Fax or carfax.
The dealer might run one for you for free.

Don't buy it if the car was involved in an accident.
A bent frame is VERY expensive to repair. Of course,
a "bent frame" does not happen in every accident but
why take a chance.

Check to see if the oil on dipstick is black--if so--don't buy it.

Consider having a mechanic running a compression check and hooking
it up to Diagnostic computerized engine tester and running the
standard tests.

Have someone start the car while you watch the exhaust--if black
smoke comes out--don't buy it.

Do a google search for Prelude to find prelude sites.

Good info at this site -- particularly on what to look for.
http://www.absoluteprelude.com/
I get around 27-28 on the highway running at 75-80.  A lot depends on how
much you run in up into the VTEC, which is around 5300.  I don't advise
running it past the redline -- 7400 -- as some Prelude owners tell me they
do. The car requires premium.

A friend of mine has a 97 SH model.  He's had an O2 sensor replaced.
His ATTS oil sensor is bad.  The connector to his driver-side (left)
headlight is now faulty as it comes on and off.  Some Honda's suffer
from leaky distributors (which are quite expensive, I assure you).

If accidents bother you, look out for that.  The hood, truck, and
fenders should still have their sticker that has the VIN.  Check all
the glasses to ensure that they are the Honda brand.

Honda's are also very specific on their power steering and coolant
fluids.  Make sure that they are Honda brands and not aftermarket.  

Make sure that you have all the factory keys.  A replacement with
built-in anti-theft chip will cost you big bucks.  While you're at it,
make sure that you get the anti-theft code for the radio.

My brother's had the 2000 base model and achieved 26mpg 50/50 split
between city and freeway.  He got 30mpg on a mostly freeway trip.