Gotta admit never seen one with the cab about to fall off from driver abuse.  I just can't imagine the operating conditions it'd take to break the cab loose. 

As Dave said, look at the hydraulic lines, there's a "few" of them,,,, and everyone of those steel lines will rattle especially when the engine's idleing,, So,, I took about 1 day quite a few feet of rubber lines and snap ties,, and every place the hydraulic line was shaking at a dead idle I insulated with a piece of rubber hosing held in place by snap ties.  Both the neighbor and I have the 1075 models with the right serial numbers.  By fluke,, his and my 1075 is less than 100 numbers off from each other. Anyway,, they've got the "big" rearends. Which is good!  It's an Eaton, I can look at the machine an give you the model # if your interested.  I've got the 800 lb weight box on mine.

There are several pivot points on the wagons, loader, 1st table- including the "edge" "flat" tip arms, 2nd table, 3rd table, and don't forget to look at the push off feet. There's replacable bushing's at all those points,, what you want to look for is wear beyond the bushing replacement.  Some guys Never grease those points thou there are zerks there. IF the push off feet are bent too much, the square holes that hold the feet will be worn oblong,, the pushoff feet don't bend unless the operator is dropping piles on the feet or pushing away from the stacks at stupid angles. Check under the 1-2-3 tables too for welds, if someone has tried to just Jam bales on the tables the preceeding table will warp some and you may see welds in "odd places". Wagon "herders" never take the time to grease them and those points wear beyond just replacing the bushings.  Look at the 3rd table forming shields, (big tall stuff that sticks up) if it's been hotrodded bad, there will be breaks in the box metal holding up the sheilds. IF repaired properly it should not be a problem.  As WYDave said if welded correctly in a timely manner, shouldn't be a problem, the problems start when small cracks are ignored and the machine is just ran till it "really breaks". 

As for the computer's pretty much either they work or they don't. That little box you can't even see under the control panel in the cab, is a big ticket item. If it were me,, I would require some sort of warrentee/guarentee on the dang thing.

As WYDave was saying, the machines are used,, so there's bound to be some cracks here and there, but shy away from big ugly repairs, especially the frames.  Broken springs are a negociable item, lot's of time's springs don't break from the initial bounce, it's the recoil on the springs that will break them. Some guys with a good shop and time in the winter, will actually mount twin shocks on the front ends to control the rocking and bouncing. Check all the spring mounts closely for welds etc.. some knot heads will yank the krap out of them trying to pull out a stuck machine.  They're not "swamp buggies" thou some operators think so...... 

Check the loader closely,, many people run the loader wayyy too close to the ground so look for excessive wear on the shoes, welds from running the sheilds into the ground, etc. Normal flat ground the loader should be at least 5-6" off the ground. (dang thing just picks up bales,, it's NOT a rock picker or rototiller !!)

If you lift the 2nd 3rd tables then shut off machine and they settle right away, they'll be needing new seals.  The main pump (sits on the front of the crankshaft on the perkins anyway) ask to have that one tested for pressure and volume. Their rebuildable, (like anything is) but that pump does pump allot of oil and can be "tired" and needing a rebuild.

Also, there's a hose that goes from the back of the head to the vaccum can on the brakes. Check that line carefully, if it's got cracks, ask to look at the inside of the line near the head, if you see dust, etc.. on the inside of the line, walk away from the machine unless they want to pay to rebuild the motor. New Holland Failed big time on that design,, they should have put a sealed air filter between the head and that line. If the line's cracked dust has direct access to the cylinders, heads, etc... 

If it's a bear to shift between gears, the transmission is about 10-12 feet away and there's linkages etc in between check for wear,(need 2nd person to run shifter while inspecting, if everything looks "ok" but still tough to shift, the shifting lever may need replacement, it's easy to replace, had to do it on my machine. That short shaft takes allot of "gaff" shifting gear forks with that 10-12 feet of "leverage" in linkages. Wagon "herders" just ram the cab shifter to switch gears instead of giving all that linkage time to move.. I usually pop the shifter linkage loose from that shaft pull up the boot and grease that shaft and receiver ball at lease once a year if not more.  

As you can tell from WYDave's and my posts,, 99% of wagon problems are the direct result of operator abuse. And, failure for whatever reason, to fix the small problems until they're huge problems. Lots of wagon operators consider filling with diesel and checking the oil as maintaince. it's wee more complicated than that, not hard to do,, just have to do it!!

Frame type issues aside,, it just depends on how handy you are, or how much you want to put into elbow grease to fix "flaws".  I know there are some wagon "rebuilders" out there, it would just be a question of quality of workmanship for the money their asking for their machines.  My neighbor was doing that for a couple of winters,,, go to California pick up a basically "good" machine, doing a quality repair/rebuild job on them and selling them locally. He made good money doing that. Take off the 3rd table forming shields, and several can be stacked up on a semi.

There's alot of good wagon's out there, but as you said just have to know what to look for.

As a suggestion,, look at the NewHolland parts book and check the parts prices,, it'll give you a good idea what to expect to pay to fix stuff some salesman tells you,, "no big deal, easy fix".....  Nothing against salesmen per se,, but I always remind myself,, their job is to Sell me this "thing", not fix it, run it, or pay for it,, they are not my "buddy" or best friend they're an aquaintce,, or someone I've just met. Their only goal is to get me to Trade my money for the "thing" that does not belong to them.  It helps me keep a purchase in perspective.