I am 49 yo, 5'10", and 240 lbs., and ride 95% in the woods. I just purchased my 3rd YZ125 in 3 1/2 years, and love them!
It will take some skills to master the 125 that you will retain forever & be able to apply on ANY bike you ride in the future. Buy as new of a bike as you can afford; there are left over (new) 2006's to be had in the $3,500 range.
Ride it stock, after first setting the suspension sag. Learn to maintain it well, then start by changing the gearing, (1 or 2 t larger rear,) adding a flywheel weight, and then finally changing the tires. Whatever you do, only make one change at a time, ride & get use to the bike after each change. There are more things you can do when you decide what you want the bike to do better.
Good luck, & have fun! Make your choice and don't look back!