Do I need to buy the whole EMSE package?

No, EMSE is modular. The ERP Bundle (Data Editor plus Visualizer) is a powerful and affordable package for discovering and confirming space-time-frequency phenomena at the scalp level, with integrated nonparametric statistics. The Basic Source Estimation Bundle adds the Source Estimator module for brain level modeling. The Locator module digitizes electrode locations, which may be co-registered with MRI using MR Viewer. The Image Processor module can segment head tissues for constructing individual cortex and volume conductor models.