Technically, any of these machines should serve you well for typical MYOG stuff (I have an Elna Supermatic, among other, and I’m very happy with it).
Personally, I would stay clear of the Pfaff just because a serviced machine that still has problems spells trouble. I don’t have an opinion on the Singer which is probably great but looks a little more complicated than what I’m comfortable opening up for cleaning, tuning or repairs. One thing you should be aware of, some of these machines (e.g. the Elna for sure, possibly some of the Necchis) need a special cam for a zigzag stitch so you should make sure that it does come with the machine. Speaking of zig-zag, some of the Supernovas were straight-stitch only so you have to identify the precise model. Flat bed machines are fine but I like a free arm which is useful if you are making or modding clothing.
In a perfect world, it really is better to buy a machine that you can inspect.