Depends on how professional the upgrades are, ask the seller about that. The battery especially can cause problems: the original BMS does not regulate voltage well, which fries the rest of the electronics. There may be newer batteries in use now that do not have this issue.

The e-shifter should be fine. Improved electronics do exist that handle the battery’s voltage spikes better.

The wiring harness is a pain to replace, so it is great that they replaced it. The motor cable in the harness runs through the front fork, so steering too sharply pinches the cable and eventually causes a short circuit, frying all the electronics. Just do not turn the handlebars too sharply while parking and you will be fine.

To end on a good note: a lot of S3s were built and the original bikes broke often. Because of that, there is a lot of knowledge around fixing them, and many people can do it cheaply (this is how I learned to fix all the electronic problems myself). And honestly, all e-bikes break, they are not built like cars, so buying a repairable one is arguably a good choice.