The most common, and probably best, advice people like you get from this
group about choosing which brand of pop-up to buy is:  find out the
reputation of the local dealers.  Are they knowledgeable?  What are they
like when the pop-up has to be serviced?  Are they responsive and do they
have good communication?  If you find one with that reputation, buy from
that dealer, whatever the brand.

Depending upon what part of the country you live in, you will need either a
furnace, or an air conditioner, or both.  Furnaces have a fairly long life,
but most A/C units are warrantied for only two or three years, and they are
expensive to fix and replace.  I am assuming the unit will come with a
"three way" (110 VAC, 12 VDC, and propane) refrigerator (also darn expensive
to replace, BTW).  An awning is very nice - actually nearly essential.  A
screen room which attaches to the awning is a luxury.  I love mine!

If the unit has brakes (I think this one does) it will need a good battery
on the front (activates the brakes if the unit 'breaks away' when towing).
A good deep cycle battery is required, and should be in good shape and hold
a charge!

You should ask about the last time the wheel bearings were inspected and
repacked or replaced (most folks here recommend that be done yearly).

Of course you should look at the pop-up thoroughly.  Does it smell of
cigarettes?  Does it open and close smoothly?  Do the brakes work (dealer
should test them for you)?  How worn is the canvas?  Do all the appliances
work, including the stove, both inside and outside?  Does it have an
electric pump for the faucet?  Does it have a hot water heater?  Is there
any evidence of any leaking anywhere?  What does the caulking on the outside
of the unit look like?  Is there any evidence of any repairs having been
done?  What kind of shape is the spare tire in?  How do the mattresses feel
when you lie on them?  (Many pop-up mattresses are thin.  Some people
replace them, or put a 3 inch memory foam cover on them.)  Most pop-ups
which sleep six have a bed at either end (check out the size:  queen or
double?), and one or two other spaces where some pillows are placed over a
board where the table folds down.  In other words, sleeping 6 adults is
often not an option.  Or tall teenagers, either.

You will need a brake controller for your tow vehicle.    Most people
recommend the Tekonsha brand (Prodigy).   A 3500 lbs. tow rating sounds
adequate, too.

I am sure others will add more thoughts.

HTH

I enjoyed a Coleman for ten years with few problems.  It had a metal
roof with a center seam.  Once had to replace the AC gasket when it
developed a leak. I understand there has been some trouble with cracks
in the one piece hard plastic roofs.  My old Coleman sat outside for
years and the only problem was dents in the roof from hailstorms.  If
you get a trailer with AC I'd invest in a device to screen the back of
the AC from such problems.  

Depending on climate I'd go with an AC and perhaps a furnace.  In
Texas the furnace would be largely a waste.  Have the AC checked.  

My old unit had a very dependable stove inside, but we rarely used it
for other than additional counter space.  In a perfect world my pop-up
would have an auxillary stove attachment for outside the trailer under
the canopy and near the front door.  Gives the option cooking outside
without having to go far.  Somehow its easier to clean up outside than
inside, and it doesn't heat up the trailer.  

The 3way fridge is nice, but usually quite small in a pop-up.  I'd
recommend getting one of those small combination warmer/cooler things
and using it (assumes you will have power available).  The 3way is
great if you get off road, something you can do in a pop-up.

An awning is pretty essential, expands  your space tremendously in bad
weather, and the shade can be very helpful.  A screen room can be
helpful, another option is solid walls.

My wife insisted finally that I get a small porta-pottie for natures
night call.  This is an option on most new units.  The older pop-ups
frequently have a convenient storage cabinet.

You may find one with a hot water heater, but they take up valuable
space in the trailer.  If it has one the trailer likely has either an
outside shower or one inside.  Again, you give up space for the inside
shower.

Since you are moving up from tent camping the mattress may seem like a
luxury, but there are differences.  Before you go buying foam mattress
pads check to make sure the slides will work properly with the extra
mattress thickness.

Thats all that comes to mind at this point.  The end of the Summer is
usually a great time to find a bargain on a used trailer.  Take your
time and look around, you can probably find a better deal from an
individual than a dealer.  

Enjoy

Folding trailers, aka pop-up campers, aka tent trailers, are "entry level"
RVs.  IMHO, Dutchmen is the entry-level of entry level popups.  Then you are
looking at used.

While walking inside the camper, if you detect soft or mushy spots underfoot,
you would be well advised to walk away.  That is a tell tale sign of water
penetration and rot.  The floor in our (bought new) 1987 Starcraft rotted-out
right from under us.  (I neglected to correct a hidden leak.)

Your NOSE is probably the best indicator of mold, mildew and dry rot.  For
best detection, you should enter the camper IMMEDIATELY after it has been
popped-up.  If you inspect the camper after it has been open and aired out for
any length of time, you lose that detection advantage.

Check ALL systems.  A gas/electric refrigerator is nice but, due to its
incredibly small size, its value is debatable.  When our 1.9 cu.ft. died, the
replacement cost was so incredibly high that I had it removed and a cavernous
storage compartment made in its place.  We always packed an ice cooler anyway.

The "bag" awning on our Starcraft was a royal PITA to deploy and stow.  I
eventually REMOVED it.  I would then re-install it for our annual, week-long
family campout.  For the rest of the season, it lived in our garage.  To their
credit, modern bag awnings are SOMEWHAT easier to use.

You are probably interested in a camping trailer because you are tired of
sleeping on the ground in a tent - a LOT of work.  An awning, screen-room or
add-a-room adds a LOT more work to an otherwise easier camping experience.

Many folks believe that an air conditioner is essential.  That was one of the
only TWO options I did NOT get on our popup.  The other was a shower.  It's a
TENT, fer pete's sake!  Good luck!

first, ya gotta learn to google...

john