Cartridge Output Level Question

Indeed, 0,3mV needs a higher gain than a 0.6mV cartridge. When his Phonosection is able to deliver >55dB gain, it is no problem.
The Reviews
You have to look what model of the 103 series was reviewed. A 103 is a 103, there are no differences, a 103R is a different model and there are a few more

He'll need either an "MC" setting/switch or a step-up transformer going into a receiver (most of which are based around the premise of using a MM cartridge where the output voltage is 10-12 greater than a typical MC). In other words, a typical MM has an output of around 3.0-5.0 mV, compared to an MC which has an output of only around .25mV. Big difference - if you try to simply plug it into a circuit for MM cartridges, you'll get a lot of noise and not much fidelity.

I would speculate that if he's using a receiver, he most likely doesn't have a turntable that's up to handling a 103. As most folks start out in analog, they'll try a 103 and typically be rather underwhelmed - the logic being it's affordable and well-respected (both of which are true, just not for the novice or casual analog dabbler). You really need a table and especially an arm that is up to the task. Look at what turntables and tonearms are being used by folks who rave about how good the 103 is for so little money - none of them would be considered modest.

The usual disclaimers apply.

had a concern that the 0.3 mV output would require him to turn the volume up on his receiver significantly

What is his concern?

There is nothing to be concerned about as long as he doesn't get in the situation where it is all the way up and not loud enough.

.

What is MC vs. MM?

MC is "Moving Coil"
MM is "Moving Magnet"

Each actually refers to what is directly attached to end of the cantilever (other end from the stylus diamond).
MM cartridges usually have an output in the range of 3.0 to 5.0 millivolts. A MC cartridge due to its design usually has a much lower voltage output (0.3 to 0.6 millivolts). The MC will need additional amplification due to this difference. Some preamps have both MM and MC abilities, some only MM. Just to muddy the water there are a few High Output Moving Coils (HOMC) that can be mated directly to a MM pre input due to their specific higher outputs.
There are many threads discussing the attributes of each design that you can search through.

Rick