First Studio Set up

Hi

Sorry I'm new to the forum, apologies if this has been asked before but i haven't found many threads.

My daughters are very musical and want to start making music.

I've got an AKAI Professional MPK Mini Play keyboard for Xmas (plus will get a reasonable Mic and monitors)

Not sure what else I need? Please can you advise?

They will be using an old Mac Pro tower with loads of RAM and Storage - do I need to get an audio interface too? Anything else?

Can tell - i am reasonable technical, just want to start on what they need before blowing too much money.
Any advice really appreciated.

A decent interface and a daw. Personally I would go for a Focusrite Scarlet interface and Logic Pro. Apple also offers a free daw, Garageband which is definitely perfect to start with as well. And if your daughters are going to do some recording they will need some headphones as well.

Old mac might have stereo audio in.

pair of active speakers would be nice too. You don't have to start crazy expensive; have a look at KRK's or Adam's.

Place to play and listen to music is a big deal. Sometimes a mic needs a preamp.