Help using VST synths live

[joeyjets]

Okay, so, I searched around on the forum, but didn't find any thread that answered all my questions (or if they did, they we're seriously outdated ).
I don't have any experience on this kind of stuff, so sorry already if I ask silly questions!

What I want to do is basically just start using software synths live. I've been playing around with different vst's at home on my PC, and totally want to bring these amazing sounds to live gigs.
I was thinking about buying an Mac laptop, as they seem more reliable then PC ones.
I would want to use maybe three or four different synths (I've already got Omnisphere, and I'm still looking for a good piano and something for string sounds), and change sounds between these.

So if I start one song with e.g. a pad from Omni, change to a piano sound from a different VST, and then back to Omni, but for a different sound, will this require alot from the computer? Will i need one of the more expensive MacBook Pro's, or will I manage with a MacBook Air?

I'm also in need of an midi/audio-interface. Both the keyboards I use at this time connect to the computer with USB, but I thought I'd buy one with midi support just in case. I would propably want to use occasionally some of the sounds from the keyboards too, so I guess they would have to send audio signal to the audio-interface? Would something like this work? M-AUDIO PROFIRE 610 - Thomann Verkkokauppa

If you have any other advice, I'd be more than happy to listen!

Thanks in advance,
JJ