This is an interesting question and you have received a number of equally interesting responses. Let me see if I can help.
Trane is a top quality brand and with a long history of first rate quality and reliability. This was substantiated in Consumer Reports a few years ago when Trane and their American Standard Brands dominated this category. The Armstrong branded units were rated in the bottom 1/3 of all of the units they tested.
The comment about the bf being an AC
tech and Trane being garbage and always needing to be fixed is an interesting one which would lead me to think that the bf in question either worked for one of the bottom tier brands and was not making a truly informed comment on reliability based on experience.
Getting an ac
condenser (outside unit) installed for under $2000 would lead me to think that they are either installing questionable equipment (not good quality) or are not installing a matching indoor coil. Recent regulations changed this year to a new minimum efficiency called 13 SEER. I would guess that the older unit that you are replacing is not a 13 SEER or higher unit and this would mean that the coil inside of your house would have to be changed as well as they will not be compatable with each other.
This makes me then think about asking where you live? This will also play a big part on the cost and subsequent value your system will provide you. If you are in a very hot climate then you might want to go above the current minimum SEER rating to help save on your energy bills. Where living in a milder climate then 13 SEER would suit you fine.
Just as a point of reference for you I live in TX and last July after extensive research and receiving 5 different quote, I had a 2 tone Trane XR13 system (mid tier with a new indoor coil) installed in my house. The coil was located in my attic with easy access (affects cost) and as it was a straight replacement there was nothing extra to do. I paid $2700 for the installation.
If you are also thinking about replacing the furnace then I might suggest they you look into a more efficient furnace 90% AFUE plus (in a colder area and 80% in a milder climate. To enhance comfort in your house you might want to ask about a variable speed blower in the furnace and a 2 stage compressor in the ac
If you have any issues with allergies or have pets in your house you might also want to look into a whole house air cleaner since the indoor air is typically several time worse that the air outside of your house.
Just my 2 cents worth. Just make sure that you are happy with the installer and their credentials don't let price drive your decision. I made this mistake a number of times in the past and have learned the hard way to avoid doing that.