Hey Fabian - Well, it won't be cheap. A 2-octave MalletKAT (which is perfect for pit work - my 3-octave MKpro is sometimes a little too big to squeeze into some pits) costs about $1,600. A 2-octave hard case (don't get a padded bag if you plan on tossing it in the back of a van or in the trunk of a car) is about $225. You'll need a keyboard stand - about $20. I think Alternate Mode also offers education discounts for their MalletKAT. You can contact them directly.
http://www.alternatemode.com/contact.shtml
Then a 13" Mac to run MS is about $1,100 w/education discount. MainStage itself is a $500 investment. (don’t get the education discount for the software – major upgrades cost extra unless you buy the full, non-education version)
So you're looking at about $3,445 to get started. But MainStage has got wonderful product support and this forum has helped me from day one. I'd still be lost in the owners' manuals or on the phone endlessly w/tech support if it weren't for the convivial and helpful MS users in this forum.
I believe Apple is committed to supporting MainStage plus, I think it is the most powerful, intuitive program available for live performance. I also think it is 100 times easier to program the KAT for shows using MainStage. If you have any other questions, I’ll try to help, and if I can’t I know someone in this forum will have an answer.
Good luck!