Currently have a Yaesu FT-50R, which was fairly easy to modify for TX
from 144-159 and 420-470. However, this great little unit was apparently
discontinued. Is the FT-60R or VX-6R just as simple to mod? Or am I
better off picking up a used FT-50R off ebay?

Haven't looked at the market offerings in a few years. Are there any
other dual band HTs to consider that are also: reasonably rugged, can
use AA batteries, 5 watt TX option, interchangeable antena, not overly
complex.