An installer gives you a work description, a scope, or a proposal page. Read it as the document that decides what you bought. Anything not written there is not part of the job. Go through the page line by line before you accept it.
Read the verbs
Look for words that say what happens to each part. Remove, replace, install, reconnect, test, and haul away are different jobs. If the page says replace but does not say remove the old unit, ask who removes it and whether that is included.
Find the exclusions
Most scopes have a list of things left out, even when no list is printed. Ask directly: what is not included? Items people often miss are the electrical circuit a new unit needs, the pad or stand, the drain line, disposal of the old equipment, and permit handling. Ask that each one be written in or marked as your job.
Check the named materials
If the page names a model, a size, or a grade of material, write that down beside the model you are considering. If the page says standard or equal, ask in writing what that means. A phrase that leaves the choice open leaves the outcome open too.
Check the start and finish conditions
Look for what must be true before work starts: access to the space, a clear path, a working water shutoff, and power at the location. Then look for what counts as finished: a test run, a walk-through with the installer, start-up readings, and copies of the paperwork left in the house.
Compare the scopes side by side
If you have more than one proposal, lay them out together. Match each line on one page to the matching line on the other. Where one page states a thing and the other stays silent, you have found the question to ask. Do not compare totals until the lines match.
Ask for a revision in writing
When a line is unclear, ask for a revised page that states it plainly. A verbal answer is a note you made, not the scope. Keep the revised page with your records for the job.
Once the scope is clear, the equipment's age still matters for planning. For a central system, the Central Air Conditioner page on this site gives typical service life and when to plan.
Central Air Conditioner: typical service life and when to plan