Any luck replacing an a/c coil?

[Steve S.]

Tenant called saying no cold air. A/c is about beyond its life. On my home I've always replaced the whole unit. This time I opted to replace the coil only.

A/c is working great now. Question is how long will the other old parts last. I spent about 1/3 of what it would cost to replace the whole unit.

Your thoughts and experiences? Bad idea? Terrible idea? Decent crap shoot?

Regardless, what's done is done.