There are two models from the 1930s, a heavy cast iron (I think) Bernina and the Pfaff 130, owners still like them but it's only straight stitch and zigzag, all metal solid work horses like the old straight sitchers. I think they look so cool, I have not yet seen this Bernina model for sale anywhere, but I want one
After WWII the free arms make their introduction; aluminum body, and a bit lighter. Berninas first is straight stich and zigzag only, it's a very nice machine. Bernina's first with built in cams is the 530 Record, it has a lot of stitches, nice machine introduced in 1954. It runs quiet and smooth, stitch quality is very nice with satin stitch fine tuning. There are a few models in the 500 number series, they are all the same quality machine but with more or less stitch patterns, and only the 530 has the buttonholer. I can vouch for the 1960s 730 Record too, I have this one at the moment, it's a favorite.
There are lots of nice models though, Necchi BU, Necchi Supernova. The early Elna Supermatics are worth noticing, they have a bit more advanced stitches than the Bernina due to the double cams. Pfaff have nice dependable machines from the 50s and 60s too. The blue Husqvarna models from the 50s are often under rated, I think the model numbers are 19 and 21, there might be closly related models very similar.
Before Touch & Sew Singer's 401,403, 404, etc. they can easily be under rated. All metal machines of a standard hardly seen since (all metal gears and made to last). I think one or two models in the 500 series are all metal geared too. There are quite a few of them around and these are in my view worth spedning a bit of time and money on. Singer's T&S are a bit later, with fancy functions like winding the bobbin with out taking the bobbin out of the case, transparent bobbins. A nice example are well worth some tlc, but I haven't owned and fixed up one my self, so I don't really know. There are in plenty though. If you choose a model from the 70s quality varies more than on older machines, and there are too many models to keep count of. By the late 50s plastic gears are gradually introduced, but the know brands generally make lasting quality machines most of the 1960s. Plastic gears doesn't have to be a big worry, but some type of plastic has turned out not to last that long. I don't think they really new at the time it was made, and they took the chance. Some models will need gear replacements because of this.