Battery is kaput and previous owner said it was the alternator. But when I boost it. It starts and runs and the gauges say it's fine. But trying to restart it after idling for 30 mins it won't have juice? I don't want to buy another battery if it will fail just the same.

 

The alternator will self excite itself at high rpm even though it can fail to operate properly at lower rpms thus it can create the proper voltage at these high rpm's. The gauge should not be the sole arbiter of whether the alternator is failing, so you should measure the voltage output of the alternator via the Black cable bolted to the alternator for a more accurate reading.