You have to understand what you're getting into with one of these airplanes, and you need to do it for the right reasons.
What you are buying is an antique, plain and simple. Just like buying a 1938 Cadillac. Just like the Cadillac, you can mess around with it and take it on local trips, or even cross country trips (if you're up for an adventure). However, there is no rational justification for it other than "it's fun." And part of the fun has to be sourcing parts, working on it, tracking down obscure information, etc. And you need to be okay with it sitting for 6 months because you can't find some part, or something is being overhauled by some 80 year old guy who only works on odd Thursdays.
If you want an airplane to use, you need a 500. That's more like a 1960s Cadillac-- it can do everything a modern car or airplane can do, it might cost a little bit more and be a little bit more hassle, but probably not even that. You can make an actual, rational case, for owning it.
As for a 690 or later, those are just plain great airplanes, and just as usable as any new competitor.