before buying one, try to ensure that it is a traditional panellerd door, made with mortice and tenon joints.
loo at the long edges (verticals) and you should be able to see a rectangular mark where the top, bottom and central rails fit through slots (mortices) cut into the verticals. If these are loose, a competent joiner can remake them, but it would be a bit of a job for an amateur.
Look at the shape of the door to verify that it is truly rectangular, not trapezoid. Doors tend to sag so the lock stile is lower than the hinge stile. It can be concealed by planing or sawing, or (see above re joiner) but best avoided.
Look to see how many holes for locks and letterboxes have been cut. If you are lucky you might be able to re-use them; especially if a Chubb mortice lock is/has been fitted. The fewer holes the better. Avoid a door where holes have been cut through the tenons.
While you are looking, look at the exposed timber in the letter slot, to see if the timber is solid, or veneered on some sort of modern "engineered" structure made of bit of scrap wood or chipboard glued together. Avoid if so.
It is normal for a panelled door that has been excessively painted, to have cracks in the paint where the rectangular panels fit into the stiles and rails. This is normal and can be corrected. it is due to differential expansion and contraction in dry and damp weather. It is usually worst in the bottom edges where excessive paint collects. Vertical cracks in the panels themselves (see above re joiner - only worth it if the door is a very fine one).
poke a slot-ended screwdriver into the top and bottom of the door to check for worm and rot. These two edges are most often neglected, and most in need of protection. Don't use anything round or pointy to probe for worm, because the marks left by your tool will look like worm-holes.
Black-painted doors most often have cracked paint because they get very hot in summer sun. White or pale paint avoids this. The front doors of 10 Downing Street are black, but they are armoured steel, and swapped for a repainted one in summer, so look very smart and do not crack.