I have an 07. Check for rust under the doors, just above the bottom seam. Check for major rust on the rear leaf spring rear mounts.

If it has the 4.0 SOHC (which I believe was used in the Rangers as far back as 01-11) make sure you don’t hear a sound like a Bic lighted being dragged across a bicycle chain. It might need knee timing chain guides if that’s the case. Although, I’ve heard that sometimes if it hasn’t gone too far, new timing chain tensioners can be installed for around $100 in parts. Some people change those out as preventative maintenance. Also, check the thermostat housing. The plastic ones leak after a while. SimmonAutoSports.com has a one piece all aluminum housing for this engine. Dorman also makes one. Also, make sure the oil was changed regularly! If the previous owners let it go too far, it could’ve started to plug the passages to the timing chain tensioners, with carbon, preventing oil pressure from getting to them. Very important to keep the oil clean in this engine. I’ve heard of them going 300,000-400,000 miles on the original timing chain guides, but that’s with oil and filter (Motorcraft is the best cheap filter) changed every 5,000 miles. High mileage can be better than low, because that means it’s probably got alot of highway miles, which means alot less wear and it probably means it wasn’t sitting very much. Gotta burn off the moisture! 04-11 4.0’s had the better timing chain guides. 

If it’s a manual, make sure it shifts smoothly. I have a little bit of wear on my 2nd gear synchro. It still shift plenty good. But it chatters a little sometimes on the way in. Also try to figure out if the slave cylinder is leaking. Mine has an internal leak and when it gets around freezing outside, the clutch drops about an inch. Then I pump it hard about 40 times and it’s back up. When it’s warm out, it lasts all season. But if the slave cylinder has to be replaced, the transmission has to be removed and you might as well just do a clutch kit at that point. I also recommend (and wish that I did this, I went about 13 years without bleeding mine) keep it bleed every year or two. Get the old fluid (DOT 3 brake fluid) out and put new fluid in there! Maybe even use a high end fluid, like Motul DOT 5.1 or something that helps condition the seals, if there is such a brake or clutch fluid. They tend to be difficult to bleed, but my mechanic did it quickly and had no problems. Didn’t charge too much, either. I think he reverse bled it. It helped, but when the outside temperature got cold, the pedal was back down about an inch. 

If it shakes at highway speed and the tires are balanced perfectly and you feel a shake from the middle of the truck, it’s likely the driveshaft needing balancing. Someone told me years back that Rangers are known for having not very balanced driveshafts from the factory. I had mine done and BIG DIFFERENCE around 60+ mph! 

Automatics I’m not familiar with, but I hear after 120,000 miles or so, they’re known for having problems. Make sure at least that the fluid was changed at least every 60,000 miles or sooner. 

Miles tell you how far a car has traveled. That's it. 

How it is has been maintained, it's current condition, etc. are much more important factors to consider. And of course price as well.

If you are asking what you should be looking for, see prior posts below:

(first link:  scroll down to ‘How do I know this car is OK to buy?’)

https://carkiller.com/scottykilmer/qa/used/

https://carkiller.com/scottykilmer/qa/good-tips-on-buying-a-used-car/

https://carkiller.com/scottykilmer/qa/used-car-inspection/#post-1019