I hate to be a pill, but speaking from bitter personal experience, learn enough to know what you are looking for before you buy something.
I have
NEVER been known as an optimist, but I do disagree with this.
My first lathe was a small Sears/Atlas 109 series. In those days I was amazed to just find one in any condition and really did not know very well how to evaluate one. It did have some accessories; both 3 and 4-jaw chucks, a jacobs chuck for the tailstock, etc. I bought it for $400. At the time that was a lot of money to me.
Over the next few months I started making plastic and aluminum parts; not bad. When I tried steel, it went bad......so much chatter. I spent a bunch of time figure things out, I learned to sharpen bits, I replaced the rocker style tool post with a small quick-change one, adjusted gibs, found workarounds and the limits of the machine. I learned an awful lot from that experience. In fact if I had the choice to do it over I probably would.
Today I am very comfortable evaluating a machine. A year ago I bought a 1937 SouthBend 9" for ~$600. Is it perfect? Nope, it shows some wear, but it has really opened up the possibilities. It is many, many, many times better than the Atlas. But I can directly apply all the things I learned to the bigger, better machine.
Recently, I traded in that Atlas lathe for credit against some other tools I was buying. I got a bunch of (used) gear cutters for half their marked price. It's hard to tell exactly what I got for it cash-wise, since the store could play with their profit on the gear cutters too. But in the end, it was definitely worth it.
If there is a "trick" it is to set expectations properly.....and that is different for everyone. It is influenced heavily by what jobs you'll be doing, what materials you need to turn, the condition of the candidate lathe, the time and skill you have for re-building, the machine prices in your area, your finances, etc.
Sorry to the OP, this does not add up to a hard number, there are so many variables......
-brino