There will be compatibility concerns. Frequently MTB frames will not clear a road sized chainrings, and often use 73mm bottom brackets which will not work with the crank off your tri bike. You will also need to source wheels for this frameset and the chainline will be wrong which may or may not cause annoying drivetrain problems.

This is a bad idea. Buy a MTB with sufficient quality components. Cheap suspension forks are heavy and are OK for reducing the impact of potholes and the like but mostly suck off road. With a MTB tire do you really need it around town?

Also I'm totally being a jerk but you make 60 sound like a death sentence. I ride technical singletrack with guys who have over a decade on you.