This thread has become a dog's breakfast of conflicting and/or redundant suggestions. (I just wanted the chance to use the "dog's breakfast" metaphor.) Anyway, get something with a mono switch to start with, and then decide for yourself if you like listening to mono LPs in mono mode, or not. I predict you will prefer it. By the way, a mono mode selector switch is typically in the linestage or high level circuitry, so stand alone phono stages will not usually have such a switch, but there are exceptions to every rule.
Uber, On the perfectionist level, you ask a good question. The result of using a mono cartridge into a stereo LP begs a few more such questions. For all intents and purposes, if the mono cartridge provides 4 pins, 2 each for a left and right channel, then the result should be similar to using a mono switch. For a 2-pinout mono cartridge, you'd have to make an effort to deliver signal to both channels of a stereo input, like a Y-adapter. If you use a Y-adapter, then each channel of the phono stage looks back at the output Z of the cartridge in parallel with the output Z of the other channel. That could have a small effect on tonal balance. I'm getting a headache thinking about this.