well, I'm assuming its a 10 seer unit. Certainly not the most efficient but could be worse. Whats your climate? If it works fine you can continue to use it and think about upgrading to the highest efficiency single stage when your ready.Do you have natural gas?

If you want to run it in heating mode you can test the temperature at the register. When in the low 40's outside you should have atleast a 25 degree temperature rise at your vents. Thats after its been running a little while ofcourse (maybe 10 minutes). I get about a 28 degree temperature rise at my vents when its 42 outside.

What that means is its 68 degrees in the house and after the heatpump has been running a while the temperature coming out of the vents is 96 degrees after its been running about 10 minutes.

10 SEER isn't even made anymore.
For the hvac industry to go from 10 SEER
to 13 SEER as the least efficient unit made
tells you how inefficient 10 SEER is.

useful life of units is from 12 to 15 years
with less efficiency as unit gets older.

best of luck

still

this was a nice single stg system at time of purchase.

three ton size, 10 SEER, and probably mid 7s HSPF rating when new.

while you have the mfg dates, you might ask when system was actually placed in service. of course, it is probably out of warranty.

I would try to find out the service history of this system along with any related problems. If it was my system, I would let the same dealer provide the PM and checkup.

you might want to know the size of the heat strip. What is your location?

these units were workhorses. expected remaining life could be 5 to 10 yrs. of course, you could have a major failure upon startup.

these are my minimum specs for a new HP system. both outside and inside units should be replaced to have a properly matched system.

15 SEER, 12.5+ EER, 9 HSPF
best matching VS air handler
full BTUs in both cooling and heating for your rated size
R-410a refrigerant(same as Puron)
scroll compressor preferred
electronic demand defrost preferred
thermostat with "dehumidify on demand" feature
staged backup heat strips
new and correctly sized refrigerant lineset

you want a thorough inspection of your ductwork system. size, overall condition, supply and return lines, insulation qualities, leak test, etc.

any hot/cold spot issues in your home should be addressed.

I would only use authorized dealers for the various brands that provide quotes. see mfg websites.

I would look at Trane/AmStd,Rheem/Rudd,Carrier/Bryant.

I would not purchase a new HP system that did not have electronic demand defrost.

IMO
Good Luck!

Thanks, all, that's a big help. I'm near Seattle, so don't need/want ac, and on an island with no chance of natural gas, but propane (and wind, lol). Sounds like even if it works for now, which I may find out today, perhaps time to start figuring out the next generation.