Sounds to me like you should be most concerned with low latency performance.
How low you can go ultimately depends your computer's CPU ability. so if your demands on mainstage are mild then you will run at lower latency no matter which brand of interface you use. If your demands of mainstage are higher then the quality of the interface's driver and the CPU overhead the driver imposes is going to impact how low you can go with latency.
MainStage is an awesome Mac app that makes Logic Pro's plugins available in an interface optimised for live playing. But it means you have to take your precious MacBook Pro on the road. Could your smaller, cheaper iPad stand in for the job? Realistically, a dual-core Mac Mini is more than capable of running dozens of instances of MainStage’s stock plugins. With proper programming (aliasing, minimizing insert FX, using global reverbs and delays, etc.), the Mac Mini can even run a few third party plugins.
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Focusrite do not have a good rep in this area. MOTU has a better rep and best of all are RME and Metric Halo with maybe Apogee up there as well.
you will be lucky if you can purchase any of those better brands smallest models within your budget.
ANother solution is to purchase an optical to analog converter box instead. This box effectively uses the macs built in audio interface but puts better quality converters on the end than the built in converters. I've had great low latency performance from a mac going this way.
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if you have a recent MacBook Pro and decent ram you should expect to run at a 32 sample buffer setting in main stages driver settings.
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If you needed midi ports you purchase a 1x1 interface for very little money.
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kind regards