All good answers above.

My suggestion is to ask the seller to have you help them assemble the
glider. This will be telling. It will tell you if YOU will be able
to assemble the glider easily and two people can do it, or if it too
difficult and needs more than two people. It will also tell you how
good the trailer is. To understand the continum of easy to difficult,
help those in your club assemble their gliders. This happens often
for glass (once a flight), but not very often for non-glass (once a
year). Or bring along someone experienced who will give you a rating
1-10. That is a good idea anyway to spot things that you might note
(blinded by visions of a new glider).

Hidden expenses - Not many. Insurance (get a quote) and annual
inspection is obvious. Tape, polish, etc is cheap. What might be a
surprise? An instrument that isn't working. They can be expensive
but this happens surprisingly seldom. But how can you test when you
are on a buying trip and the glider is on the ground short of a test
flight? (unlike powered, you will almost never get a flight before
buying unless its a two place) Answer, you basically can't. So get
the seller to guarantee the instruments are good. Most sellers will
not have an issue with this.

Trailers - Someone told me that the first glider you will buy for the
fuselage and cockpit, and the second for the trailer. For the $10K
range, you will most likely not get a great trailer. The assembly
test above is key to seeing if the trailer is good enough for you. I
love my trailer and its a homebuilt! The only problem is that it is
heavier than the nicer fiberglass/aluminum trailers. How mechanical
are you? You might be able to take an iffy trailer and fix it up
(easier than fixing up an iffy glider's airframe). I went from a
trailer that was OK to rig to one that is really quite easy to rig by
adding a few innovations of my own.

Automatic control hook ups - For $10K you probably won't get this. I
have a DG-101 and it has hotellier. Not too bad. Again, a trial
assembly is useful. DG's are basically easy to rig and easy fly.

My $0.02.

Good Luck,
John DeRosa
http://aviation.derosaweb.net/dg101