A true shotgun mic is highly directional but this can actually cause serious problems when recording indoors in a small room. In such an environment, there will be lots of frequencies reflecting off of various surfaces hitting the mic at odd angles. Off-axis noise and shotgun mics can cause some weird, undesirable effects. So with that in mind, I'd stick to either a standard dynamic mic like the SM58, or a condenser that has what's called a "cardioid" polar pattern (some condensers also have a selector so the user can choose the pattern).

The shield definitely won't completely block out ambient noise, but it will help as long as it's positioned properly.

The audio interface is what converts the analog audio from microphones and/or instruments into a digital signal that the computer can understand. They also have built-in preamps to boost that signal, as well as 48v phantom power which is required by most condenser mics. For the absolute best quality, an interface is necessary BUT pretty much anything is better than what he's got now. I just found a great deal on a Rode NT-USB mic bundled with an isolation shield. This mic essentially has an internal interface, so there would be no need to buy a separate one (or XLR cables for that matter). I personally am a huge Rode fan and primarily record my vocals on a Rode NT1. Also, this Rode/Focusrite bundle is a killer deal.

I'm a Mac user so take what I say next with a grain of salt: If his phone or tablet is USB-C powered, you could go with a USB-C interface. I use a MOTU M2 and can plug it directly into my iPad or MacBook Pro. I assume it could be plugged directly into an Android phone or tablet, but you may need to do some further research to know for sure. You would also need to look into Android DAWs (Digital Audio Workstation) because frankly, I just don't know enough about them to recommend any.

I've heard the MIDI has nothing to do with audio. It's just a way of recording various actions such as note on-off, velocity, pitch etc,. Someone said I would need to use 1/4 cables to plug into the back R and L audio output of the mc303 and the others ends into the 1/4 inch jacks in the front of the MOTU M2 interface. Is this incorrect?