There is an old saying that "they don't make things like they used to". It's true that there were many fine pianos made in the early part of the twentieth century but there was also a lot of junk made as well. Given many decades of use and abuse some pianos will have stood up remarkably well while others are now only suitable for the local landfill site. High quality pianos are put together with better materials and workmanship and are meant to "last a life time". It is always advisable to look for these high quality instruments as they are far less likely to have developed problems over the years. You may think that you don't need a great piano because you are just starting out, but if you get stuck with a piano that won't hold a tuning you may never get past the just starting phase. No one ever regretted buying quality.
Generally speaking when it comes to pianos, bigger is better. There is no substitute for size when it comes to sound. Everything smaller than a nine foot concert grand is a compromise. However most of us don't live in homes the size of concert halls and the price of a concert grand is out of reach for all but a few. That being said the principal remains, bigger is better.
The reason behind this is the physics of sound production. Longer strings produce more depth of sound. People frequently refer to smaller pianos as sounding tinny. That's because shorter, stiffer strings tend to produce more high harmonics and less of the fundamental or bottom or the note. The sound from longer strings also tends to last longer before it starts to fade because they are not as stiff as the short strings in a smaller piano. This becomes an issue when playing pieces like Beethoven's "Moonlight Sonata" or hymns that require a longer sustain on each note. A smart sales person might be more likely to play something that uses a lot of fast short notes like a ragtime piece when showing off a small piano.
The soundboard in a large piano is also responsible for the sound it produces and a larger soundboard will produce a better quality sound. The soundboard in a smaller piano is less flexible and produces a thinner sound.
Pianos tend to show their age in the bass section first. The bass strings are made of a solid steel string with copper or steel wrapped around them to produce the low notes in the scale. Typically as the piano ages the bass strings will start to corrode and cause the bass to sound thumpy. By the time a piano is about 30 years old this starts to become apparent and they may need replacement. Some higher quality pianos however have bass sections that remain lively sounding for many decades longer.
The next area that should be looked at is the condition of the hammers. If you lift the lid of an upright piano and look down at the hammers (these are the parts that strike the strings) you can see if the hammers are worn. If a piano has been played a lot over the years it will have developed deep cuts in the hammers from contact with the strings. Some hammers have actually flattened out to the point that the piano is starting to sound like a honky-tonk piano. This is caused by the flattened hammers slapping the strings, instead of bouncing off of them as the original rounded hammers would have. If the hammers are too flat they will have to be replaced but if they are just slightly flattened, a piano technician will be able to file away some of the surface felt to restore the original shape.
It is beyond the ability of most consumers to assess whether a piano has loose tuning pins but if you are looking at a piano that is desperately out of tune don't assume that it will be just fine once it is tuned. Some pianos have loose tuning pins and can't be tuned properly without replacing all of the pins. This can be an expensive procedure. Some pianos even have cracked pinblocks. The pinblock is what holds the tuning pins in place. If it is cracked, repining is inadvisable. It is possible to replace this pinblock but it is too expensive to be practical for the average upright piano. If a piano sounds like it is really out of tune, it would be advisable to have a piano technician assess it before buying it. (This is a good idea most of the time).
If you are looking for an older full size upright piano I would recommend the following makes;
Steinway, Mason & Hamlin, Chickering, Knabe, Sohmer, Baldwin
Please bear in mind that pianos don't last forever. Many upright piano made by these companies before 1930 were very well made and have held up reasonably well but the older a piano gets the more likely it is to have dead strings, worn out hammers, brittle action parts, soundboard problems, loose tuning pins etc. For this reason I don't recommend purchasing a piano from the late 1800's. If you are in the market for a smaller upright piano ( 48" or shorter) I would recommend looking at the more recent Asian imports over Canadian and American made uprights from 1930 onward (with the exception of Steinway & Sons and Charles Walter and the larger Baldwin uprights). Most North American built small upright pianos were not well made and don't compare well to the imports from the 1970 to the present. For small uprights I would recommend the following makes -
Young Chang, Samick
European upright pianos are less common here in Canada. You are not likley to come across many in the used piano market with the possible exception of Petrof which would also be a good choice. In recent years there have been a number of different Chinese made piano brands that have appeared in the Canadian market. Many of these are reasonably well made, by that I mean better than what was being produced in Canada and the U.S. in the 1950's, 60's, 70's and 80's.