Can I add an external amp to a mid-range AVR later?

Yes — most receivers in this list have pre-outs (the Denon X3800H, Onkyo TX-NR7100, and Sony STR-AN1000 all do). Entry-level units like the Denon AVR-S570BT and Yamaha RX-V385 generally do not, so if you think you'll eventually add a separate power amp, prioritize a model with full pre-outs from the start.