The most common and cheapest units are based in the old ICL7106 chip.
Just be aware that, without some additional circuit, the ICL7106-based devices cannot measure a voltage which is referenced to its own supply common.
The ICL7106 is obsolescent. Not for performance, but cost, convenience, cost, price and cost.
The most common and cheapest panel meters are based on 8 bit microcontrollers. They use their internal 10 or 12 bit ADC with lots of averaging to display 3 or 4 digits of resolution at a 1-5 Hz refresh rate. More accurate ones have an internal bandgap reference that they correct against, but plenty of them just use the supply 7805 as a voltage reference (which admittedly does provide generous dithering noise for input averaging).
These are readily available in 2 and 3 wire models. If you do find a cheap 4 wire meter it's probably using pseudo-differential inputs. That's good because it lets you eliminate the current draw of the LED display from the reading and doesn't require an isolated supply. It's bad if the input range limit hits you.
You can still get the typical old-style four wire meters that require an isolated supply, but what you think you are gaining in accuracy might be defeated by the noise out of a typical DC-DC converter. It's not they have a lot of noise, it's that the remaining noise can be really difficult to cheaply eliminate. And with the variable frequency and pulse width, it's difficult to be certain that you won't hit a bad regime.
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