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Depends upon if you want to get into the replacement work in addition to service.
You are the first one there and have the opportunity to take care of the change out.
Your paid for service call will let you examine the job requirements at length and put you a step ahead of the next estimator. Some will deduct the service charge if they do the install.
You will probably work cheap as the first few jobs will take longer than later ones.
My nephew has a small van and does both. He has a small trailer he pulls to installs.
You could have ductwork made for each job if needed.
OR, I have been able to make most ductwork needed with a "Hensler Bender".
Probably $50 and has paid for itself many times. Will do up to 36" tall plenum.
Very simple tool, instructions are printed on it. Actually makes pretty good looking bends.
I make them on my shop floor as needed. Great exercise BTW. Close up the box with some S drive.
Most other fittings can be brought from supply houses.
You probably have all the tools for AC service/install.
The duct tools are relatively inexpensive.
Mine all fit in one single handle tool box. ($600-700 worth)
Also includes 12-18-24" folding tools.
Eventually you will want an electric tin snips and Snappy circle cutter.
An interesting one man shop show on Utube is Steve Lav.
He does quite a lot with minimal tools. Mostly service but a few change outs.
I have been in business for over 40 years, almost retired and actually learn a little tip from him on about each show.
He has no more tools than I mentioned above.
IIWM, I would go for it....Good Luck!