I have a 20 yr old Trane XE78 furnace coupled with some off brand a/c unit. This combo was original from when the house was built in 93. System is functional right now and it has been trouble free, but I have a strong feeling the a/c unit will go tits up this summer(sounded like it was laboring to run last fall) and I want to beat it to the punch. It makes sense to me to replace the furnace along with the coils and condenser, line set, etc, until I got the quote for a new Trane XR80 furnace and XR13 a/c installation. Now I am wondering if it makes sense to spend 4500 to replace a functioning system just to gain piece of mind. Any thoughts on this would be helpful.
Also, If I do go for it, Are there noticeable differences between reliability of equiptment from one brand to the next? I like the Trane - would this be a decent choice over others?
Thanks in advance for any opinions.

Obviously replacing the coil is easiest when you replace your furnace. I'm sure there are some reasons to replace nice working coils and line sets when you replace the other equipment (just not convinced of any reason--yet, unless you are changing the size of your ac unit). I'm a fan of mass produced single stage units like a Goodman, that doesn't have hard to find parts. Sounds like you like your equipment (and hope you like your contractor). You've done your homework! Why not just swap out your condenser and have the lines checked-out to see if they are still in good condition? Wait till the furnace dies; and by then there might be some government incentive to upgrade?

4500 dollars for a complete new heating and cooling system is extremely reasonable. Just try to find a lessor quote. Bet you cant. The main reason for replacing the complete a/c system (condensor, lineset, evap coil) is because R-22 is being phased out. As of 2010 R22 components could no longer be manufactured (there are some loop holes, like some manufactureres are still making r22 condensors, but they come precharged with nitrogen, not r22, so when you figure in the cost of the unit, and the cost of the refrigerant that needs to be added, it gets a bit spendy , especially with the price of r22 to today.) and R22 refrigerant will no longer be made as of 2020. Basically, R22 is going the way of R12. And you can see it in the price of the refrigerant and components to date. In a new system these days, you will be looking at R410a.

Now, if your current system is working just fine, I have a theory of, if it isn't broke, don't fix it, and don't replace it. Just start setting some money aside so that when the system has a major component fail, you can then replace the complete system rather than dumping money into an old system. I have sold and installed Rheem, Goodman, Train, and now am installing York. Most new brandname units are basically the same (warranty wise). It really depends on what your local hvac shops install. You don't want to buy a brand that isn't installed locally, or you will not have quick access to parts if the unit fails. I have actually worked on a few Trains, and they seem to be of great construction and components. However, if you buy a Train, or Lennox, you will not be able to buy parts from them other than through a certified dealer. And most certified dealers will not sell you the part unless you pay them to install it.

Overall, most namebrand units these days are good. The most important part is reliability of the installer to come back if ever there is an issue.

The quoted price is very reasonable. Depending upon where you live, maybe upgrading to a 2 stage 80% or a 90+% efficient furnace might be worth your additiional investment. If you live in an area where the A/C takes the heavy load, stepping up to a 15 SEER might be worth looking into. I really like the two stage furnaces but not with a variable speed motor. The motor alone can easily cost $500+ to replace. End of sales pitch.
I agree with the others on replacing a working system. If it ain't broke, don't fix it unless you have very good reason to do so.

Right on target, Grady. I don't mind two stage units, but I am not a fan of variable speed (not to be confused with multiple speed) motors. And when you get into modulating units, replacement parts prices go through the roof. Just not worth the investment in my opinion. Goodman two stage units are good. York single stage (at 95.5 eff) are good too, as is any single stage, or two stage unit that has a rating or 95% or better. Steer clear of any units with "smart valves" or modulating motors.

Anybody still use smart (jshould have been called dumb) valves?

Ive heard carrier, heil/tempstar, and arco air still use them in some of their models (dont quote me on that). I agree, them things suck. Great idea, huh? Lets integrate a complete control board in with the gas valve... that should work great, now instead of merely replacing the gas valve or control board separately, we can stick people for the price of both if one or the other fails, and charge an arm and a leg for it to boot.