I just replaced one of the systems in my home, it was 19 years old.
I've had a couple minor issues the last 2 summers and thought it was time.
It is typically not cost effective to rebuild an electric motor under 1 hp, $1400 for a motor is crazy.
You could change out just the air handler [unit in the attic] it would need to match your condenser [unit that is outside]. But, you may not get a full warranty since you are just replacing half.
If you plan to stay, I'd recommend biting the bullet and installing the entire system. Most units give a 10 year parts warranty on everything and 1 year on labor.
I did research, American Standard [made by Trane] got the best reviews. However, my local HVAC contractor was not a AS dealer, so I went with Goodman [made in Houston].
For a 3 ton unit [serves the 2nd Fl in my home] I paid $4500 [received 2 bids for same price]. It was a real b!tch to get the new air handler in my attic, the jump hole is only about 2x2'.
Been cool ever since and no regrets.
BTW - the unit I replaced was a Payne, I would not recommend.