IMO a .22 is plenty powerful enough, but the greater the accuracy of the shot placement, the less power is needed. Accuracy depends not just on the accuracy of the gun and pellet, but the shooter. I have a low power tune on my RTI since I'm shooting in the city & try to minimize pass-throughs (thank you Orion).
If you're just dealing with iguanas outside your home, a good quality springer would avoid the extra expenses of the pcp world. It is fun, but not without cost.
Here's a thread on iguana hunting that you might find helpful.
https://www.airgunnation.com/threads/iguana-minimum-acceptable-caliber-fpe-kill-zone.1273193/