welcome. this thread has lots of data points you may find useful.
to get you started,
id looke at no earlier than a 2006. that year nissan made some nice improvements.
first gen armada had manifolds that would crack easily. there was a recall on the petrol tank.
test the rear AC and make sure it works. first gen armadas the ac lines ran under the car and there are some points in that run that can wear out due to rubbing on the under chassis, requiring the car to be lifted slightly off the frame to replace those lines.this would only be an issue if you sourced an armada that was in the rust belt/northern areas as salt and slush can corrode these components. there's threads on here you can search if that matters.
i had two armadas and one qx (2006, 2008, 2012) all new when i got them and they all gave me worry free use. i had no major issues with any of them and i drove them like a rental.
at the end of the day, no matter what you get, the car is only as good as its maintenance and previous owner(s), so look for maintenance records. look for rust. if you have a mechanic you trust get them to smoke it over.
by and large the armada (and qx) are very reliable cars. many cats have had them upwards of 300K miles and more.
EDIT: the 2007 is cosmetically better than the 2008, but again, check the maintenance records on them. if you can take either one to a shop to check for obd codes that would be ideal. its not much to go on off just photos.