What models do you like? I’ve seen high mileage mustangs from the 1999 up. Really they all have stout V8’s. The coyote motors from 2011 and up are really strong motors. I’d grab one of those personally. In addition there are big differences in horse power over the generations.
1999-2004 had 260 hp. 2005-2010 had 310 hp. 2011-2014 had 420 hp. 2015 to 2017 had 435 hp and the 2018-2019 has 460.
Something else to consider is that 2011-2017 V6 had 305 hp and gets between 25-30 mpg. They take boost well, come with forged internals and a limited slip diff. The performance package for that V6 is pretty sweet too. Finally the V6 motor was designed by Mazda and goes 200k+ easily.