TL;DR: Yes, vintage horns can be OK. Below are a few things to beware of.
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Personally I would be reluctant to spend thousands of dollars on a new instrument when there are so many fine used instruments to be found. A good used R13 will have all the problems worked out of it and will be past the vital first year or so when cracking is most likely. Also, although I'm sure I'll collect some flack for saying so, I don't believe that the R13 of today is of the same quality as the R13 of 20+ years ago. While there may have been technical improvement along the way, some essential aspects of quality have been lost. If you're not sure about getting the right instrument for you, there are several services around that will select a horn for you, leaving you to make the final choice from several examples. Just my thoughts.
I like a lot about this answer. I like a lot about Helen's too. I'll try to combine and synthesize.
Going back to saxophone for a minute, one of the reasons why some players look to vintage is because the best of those saxophones were made "back in the day." As an example, I'll use Helen's Martin Committee "III": that's the last professional model saxophone (they had one with more bells and whistles, but it was still the same horn) that Martin ever made. They stopped making them in 1971. So, there are no better Martins out there. Buffet still makes clarinets.
First, an opinion: if you're ready for a professional-quality horn, nobody can dispute your choice. You play-test some and buy the one that best suits you.
Second, "pre-R13" generally means very old. You might start running into problems with that "high pitch" thing: "high pitch" was an old intonation standard where A=457hz. The current standard is "low pitch," where A=440hz. There's also a common orchestral standard in France -- and Buffet makes some horns in this pitch -- where A=442hz. You will not be able to play any high pitch woodwind in tune with a modern horn and high pitch woodwinds cannot be made to play in tune with modern instruments. You can play A=442hz instruments in tune with A=440hz instruments (and vice-versa) if you listen and lip down a bit.
Third, I mentioned that Buffet still makes professional horns.
Buffet would say that their current R13 is better than the ones made in the 1950s. Period. Full stop.
However, I completely accept Tony's comment that horns made at whatever point could have been the best the company made at that time, for whatever reason: they had more money to spend on R&D, they used better materials, they had better quality control, etc. However, this doesn't necessarily mean anything. If you play a 1950s R13 or a low pitch "pre-R13" and think it's better than the new horns, go with the old one -- provided the old one's not damaged in some way that you can't get fixed.
For me, when I decided I wanted to buy a baritone saxophone, I had a choice of Yamaha and Selmer -- all that my local store carried back in the 1980s. I didn't want as Selmer USA student horn and I didn't want to pay twice as much on a Selmer Super 80 than on a Yamaha. After play-testing, I thought the Yamaha 62 was great ... but it wasn't a couple thousand $ better than the Yamaha 52.
Finally, let me add in a bit of SOTSDO: you should spend some time looking for a nice mouthpiece and ligature. The mouthpiece is more of a concern than the ligature unless the ligature you're using is damaged. 3rd party bells and barrels? Those are out there, but they're not as much a concern as a good horn and a good mouthpiece.
PS: Do remember some schools and colleges are stupid and require you to get a specific make/model clarinet and/or mouthpiece. You might want to check on that before you buy anything.