Marco,

Why do you want a truck?

If you are not hauling full 4x8 sheets of plywood, or something similar on a
regular basis, you probably do not need an 8 foot bed.
I am not even sure why they still sell standard cab trucks :-) there is no
place to keep your crap....  I suggest you get a extended or crew cab.
I also would not own a 2x4 truck again, I have driven 4x4's since 1982.

Good Luck

   What do you intend to do with it?  If it's towing, what weight/length of
load, over what distance, terrain, etc?  Do you intend to use the 4wd
on-road in bad weather, or just for plain off-road?  These answers will
help.
   I just picked up a 95 F-150 w/ E4OD automatic, 4WD, and 5.8L engine.  It
has 150k miles, and runs fairly well.  It was $4k - but most we considered
were around $5k.  Definitely use all of that $6k budget - but stay away from
dealers/resellers since they usually sell trucks for ridiculous resale
values.  We use ours for towing a horse trailer weighing 5-6000lb - and
although it's 1200lb under the recommended maximum, it's a live load that
certainly is a test.
   Do extensive road tests, and make sure you have highway, some twisty
roads, and some soft surfaces (or with snow on the roads).  Obviously, don't
engage the 4wd on any surface with any measure of traction - so you may not
be able to test the 4wd at all.  Flip the switch on the dual gas tanks and
be sure each operates independently.  (The valves and pumps often fail,
pushing all the gas into one tank).
   Look under the truck after the test drive and look for leaks around the
tranny, oil pan, transfer case, etc.  Look for leakages where the axles meet
the differentials.  Look for leaks around the gas tank.  Do your best to
evaluate if the balljoints, steering, and suspension linkages are fairly
tight.
   Good luck (although you probably bought it since I'm replying to an old
post)

   Dave