I have serviced a shimano freehub in the past - it is possible but extremely fiddly, done more to confirm suspicions on the failure mode. The pawl was sticking in old dried up grease, and entered into permanent freewheel mode on a chilly ride, seals and bearings were in good condition. Rebuilt unit is in the spares box.
If new parts are available buy them. Taking old ones apart is an interesting experiment for an adventurous mechanic, if any sub components have failed they are not available, and they are a real pain to re assemble. Also really needs an old hub to bolt them onto and mount in a vice.
If there is significant play between freehub and the hub, usually with brown residue, the bearings are shot due to water ingress. Once the bearings develop play the seals cannot be salvaged or replaced. I have seen a few shimano style bolt on freehubs that have died this way.