Mom's buying a new AC Unit -needs help

My recently widowed mom had a company came out to do a tune-up on her

A/C using the SCE rebate program. To my surprise she ordered a new AC.

The tech said the unit was drawing more amps than it should and was therefore costing her more money to run.

The unit the tech sold her was a Trane XB13 which I'm sure would be better than what she has. But...The entire HVAC unit she has is the original that came with the house which is 18 years old, and they are not replacing the heater on the inside of the house.

I know there is some type of coil in the heater that the blower blows through,

so shouldn't this be replaced as well? I think the whole unit should be replaced. Can you gain that much efficiency by just replacing part of the unit?