The question is not whether you are handy. It is what the job touches. A faucet-mounted or under-sink cartridge is a small, reversible change at a fixture you already use. A whole-house system sits on the main supply, changes the plumbing in the wall or basement, and often needs a drain and sometimes a power connection. Those are different decisions, and mixing them up is where homeowners get stuck with bad work or voided terms.
Point-of-use jobs you can usually handle
Replacing a cartridge in an under-sink or faucet unit is typically a matter of closing the local shutoff, releasing pressure, swapping the cartridge, and checking for leaks. Keep a towel and a bucket nearby. Check that the shutoff under the sink works before you need it, because a stuck valve turns a simple swap into a flood. If a unit needs a new housing, a new valve, or any new pipe, treat that as a plumbing job and reconsider.
Whole-house jobs that usually need a licensed hand
Work that cuts into a pressurized main line, requires soldering or new fittings, adds a bypass loop, or wires a controller into an outlet or panel is usually more than a homeowner should attempt. Ask the provider exactly which connections the install requires and who is responsible for each one. Ask whether a permit or inspection applies at your address. Check with your local building office, since rules differ by place, and a provider's answer is not a substitute for that check.
Read the warranty before you touch anything
Some warranties require professional installation. A homeowner-installed unit may lose coverage even if the work looks correct. Read the installation section of the warranty and the quote before you order parts or start any work. If the provider offers a hybrid option, you may be able to do routine cartridge swaps yourself while a pro handles the initial installation and any later changes to the main plumbing.
Questions to settle before you decide
Ask the provider what the quote includes beyond the unit: fittings, the bypass, the drain, the permit, and the first media charge. Ask what they expect you to do yourself after install, and whether they will show you the steps on the day they install. Ask how a homeowner-performed swap affects the warranty. The answers will tell you which parts of the job you can own and which parts are worth paying someone else to get right.