You don't HAVE to buy additional sensors but those who run a Haltech run a HALTECH for FLEX reasons.flex tunes want boost and ethanol, so you'll need the content sensor and the 3 bar map sensor which is exactly what you just mentioned.
Add them to the price as well.
temperature and pressure sensors & O2 wideband kit is also more again if you want to use haltechs wonderful features you'll need all of the senses to read them obviously just stating it doesn't stop at $2100 to fully utilise it's features.