There isnt a minimum power level for the shooting range... you can enter the "Normal" shooting range at power level 10. Launching the shooting range will force you into the highest difficulty you can currently access in the portal... but you can tune that down if you like. The game probably doesnt let you turn it up, cuz they want your testing to reflect the same difficulty you'd experience in real activities. All the modifier options in the Shooting Range will reflect which difficulty level they're associated with in the Portal. For example, you wont be able to access Brawn in a -15 Expert Shooting Range cuz its not even available in Portal activities till you unlock GrandMaster at -25 difficulty.

Difficulty unlocks in the Portal are tied to your Power, so they copied that over to the Shooting Range. You can't run Ultimate difficulty content in the Portal without being 451+ (either yourself or via Fireteam Power), so they aren't letting you go into the Shooting Range* on Ultimate difficulty unless you're also 451+. Same applies to lower difficulties like GM or Expert.