I was just wondering what level a 6 person party could reasonably reach in the course of the normal game in IWD2 (no HoF) by playing normally. Just wanted to know if I get to try out ninth level spells in the normal game or have to wait for HoF mode after the normal game ends.

The average party level will be a little lower than 17.
In my party game the aasimar was level 16, while the non-ecl races were level 17 at the end.

Yep, same here. My party of 6 had two ECL races (one Aasimar and one Tiefling). The ECL races were level 16, and the others were level 17.