In my limited experience, vintage sewing machines are rather like cats - they choose their owners rather than the nominal owner having much say in the decision.

My wife was given her Pfaff 262 by a friend.  My wife wouldn't trade or sell the 262 for any newer machine - it suits her very well.  The 262 is a stodgy grey machine - its beauty lies in its abilities, not its appearance.  The 262 is a zig-zag machine, with some fancy stitches to go with the plain ones.  It is also fast, powerful, and damn near silent.  I wouldn't use it to sew leather like I do with my Adler, but mostly because my wife would not be amused by the changed thread tension settings it would take to do leather.

My daughter mentioned handsewing a blouse (despite owning a modern sewing machine,) so I sent her a picture of her greatgrandmother's Pfaff 31 and asked if she'd like to use it - love at first sight.  Her modern machine is a typical beginner's machine - zig-zag, inexpensive, plastic, noisy, slow.  The Pfaff 31 is straight stitch only, treadle powered, quiet - and beautiful.  She's going to use it to make a stuffed (plush) animal for a friend's infant child - as soon as her university exams for the semester are completed.

My Adler was a gift from my kids - they bought it because it was inexpensive and the seller said he used it on leather.  It was filthy and sad looking - and I found it irresistible from first sight. I've spent considerable time cleaning it and getting it back into a state where its beauty really shines.

Between them, those three machines represent a good selection of the reasons to own and use a vintage sewing machine.

The Pfaff machines are common here - we live relatively close to where the Pfaff factory stood before the company was sold off and production moved to where ever it is that things are made these days.  I can get on the eBay "Kleinanzeigen" (classified ads) on pretty much any day and find Pfaff machines in my area that I could buy complete (head and table with motor or treadle)  for a pittance - or just for the effort of driving over and taking it off the hands of someone cleaning out a basement or attic.

The Pfaff 262 is "modern" enough that I can buy spare parts on Amazon.  The Pfaff 31 is common enough that I could easily get spare parts on eBay.  Both machines are closely enough related to more modern machines that consumables (needles, bobbins, bobbin holders) are available on Amazon.  User's guides and other information is available at various places on the internet - Pfaff still provides downloadable guides for many of of its older machines.

The 262 was a professional machine for its day - made for daily use by tailors and seamstresses.  It is fast (but not too fast,) solid, flexible.

The 31 was a home machine (a domestic, as sewing machine enthusiasts refer to them.)  It is not especially fast, but it is reliable and solid - and simple to use.

The Pfaffs are the ideal case - good solid machines, common, well documented, have easily obtainable spare parts and consumables. They are what you want to look for in a vintage machine.

My Adler class 8 is pretty much the opposite of the Pfaffs.  While it is a good, solid, reliable machine, it is less than ideal in that it isn't a common machine.  

User's guides are hard to come by. 

Compatible needles are still made today for industrial machines - which the Adler 8 was in its day. It was made for factory style use - it was made to be fast, even when operated by a treadle. 1200 stitches per minute may not sound all that fast compared to a modern industrial machine, but it was doing it with nothing more than foot power.

Spare parts are a matter of poring through eBay, looking for bits and pieces and buying things because they were made for a similar machine and might fit the Adler 8 - or not, you take your chances and sometimes guess wrong.  I've got a screw I'd like to replace, but I haven't found one that might fit - the thread on the screw is some oddball gauge that I haven't been able to locate.

I managed to get hold of a bunch of (rusty) bobbins that were made for a similar machine. Nobody makes new bobbins to fit the Adler 8 anymore.

If I wanted to get a spare bobbin holder, they show up (for related old machines) now and again on eBay for about 30€ each.

The speed is a problem all on its own.  I don't have the treadle table that the Adler 8 would have been mounted in when new. I have to run it on a "backpack" sewing machine motor.  The controllers on those thing are crude, and don't allow fine speed control.  The Adler 8 runs faster on the controller's lowest speed than my wife's AEG 376 can manage on its fastest speed.  When sewing cloth, anyway.  On leather, the load is enough to bring the speed down from "insane" to "just manageable."

If you are looking to get into vintage sewing machines, you need to keep a few things in mind:

1. Make sure what you want to use it for. A straight stitch machine will be a problem if your sewing tasks need a zig-zag - but a straight stitch machine makes neat, even straight stitches that a zig-zag can't match.
2. Make sure to get a common machine for your area.  Nothing sucks worse than not being able to work on your projects because the (cheaply and readily available) parts you need are shipped by carrier pigeon from a friendly fellow in Timbuktu. The Pfaffs I mentioned are common here in the area where I live in Germany. In other parts of the world, Singer is probably a better bet. There were also many Singer copies, as well as local manufacturers. You might find a locally made machine that has great support in your area.
3.  Make sure that consumables are readily available. Needles, obviously, but also bobbins, bobbin holders and drive belts.  Many older machines use simple leather drive belts that you can buy almost anywhere while later machines may need custom made toothed belts that are hard to get.
4. Make sure the machine matches your abilities.  A high speed industrial machine is probably not a good choice for a beginner (like I am.)  Likewise, the pace of a reliable (but slow) domestic may frustrate more experienced users.

Even if you do your best to make sure you get a practical vintage machine, you may find yourself mesmerized into taking possession of a beautiful old oddball built of unobtanium because it picked you out as its new owner.  If that happens, all I can tell you is to relax 'cause you don't have any choice in the matter - that, and "welcome to the club."