Here's an example if I were figuring up the cost of your system. I'll assume it's a 15 seer AC unit with natural gas furnace, about 80k BTUs and that the ductwork exists, is fine, and sized properly.
$3,500 minimum material costs (units, line sets, pad, disconnects, etc.). $1,000 minimum labor costs (It'll take 2 people about 10 hours total at $50/hr. Remember, I'd be paying their wage plus a host of other stuff like insurance, trucks, someone to sit in line getting a permit, etc.) $500 for the expected warranty labor I might have to provide in the future, the cost of that "free" quote, the fact I have to keep lights on at the shop, etc.
$5,000 total in costs before I make a profit. Add 20% to keep me in business and happy, and you're at a reasonable installed cost of around $6k. That's about where you should expect at least one or two quotes to come in at. If the ductwork is not sized properly (and its a good bet it isn't for a system 40 years old), then that's more money. Maybe the extra $1k to get you to the quote you were given.
If you did it yourself, I'm guessing you'd be at $4000-4500 (same ductwork) once you get some tools you'd need, break a few things cause you don't know what your doing, etc. You'd be looking at closer to 20 hours of actual labor, with flying spaghetti monster knows how much time researching how to install the unit.
My advice: get more quotes. It should cost you $6k. When you get a quote around $6k, hire them. Use your time finding new real estate deals and factor that cost into any future decisions on those new deals.