For Music Streaming you don't need a strong PC.
All you need is A soundinterface to connect your instruments/mics to the PC
A webcam
It's beneficial to have multiple Monitors (one for Chat and to monitor the stream an other one for notes/content)
So you may want to buy an additional cheap graphic card. to connect that second monitor to your pc.
An i3 should bring enough processingpower to perform all the tasks. Go for the i5 if you want to have some headroom
If you want to use one of these expensive high quality Kameras you may need one of this expensiv capture cards.
But that's most likely over the top.
If I get you wrong here and they want to stream games, it would be good to know which type of game. (A point & Click adventure will do just fine on old machines but to stream a game liek Crysis, you need a strong rig...)
Rule of thumb for streaming games is look at the recommended specs for that game, and get an even better PC, as streaming will cost you a fair amount of fps.
If you intend to stream titels like Star Citizen i would highly recomend to buy 2 PC's.
One for actualy playing the game, and a second one with a capture Card to stream the game.
