I'd ask the company what they recommend. They must have many representatives with the same requirements so they should know, both for the laptop and the printer. Moreover, it has to run their software.
If the clients don't happen to have WiFi in the middle of nowhere, or don't want to give their WiFi-password to a passing salesman, he'll have to store the data on the laptop and send them to the company from home. And maybe it's best to do that anyway. It won't harm the process if it's done every eventing, I think. So there's no need then for any wireless on the laptop (althought all have it). Or maybe they have a company paid 3G or 4G dongle to send the info from their application?
It fully depends on the clients router how secure WiFi is. That has nothing to do with the laptop (although it can be argued that a laptop with Linux is safer). having WiFi off when visiting a client certainly is most safe.
Nor is "power" (what powe?r) of any importance. So that are rather strange requirements.
And why 3 or 4 USB-ports? One for a mouse and one for a printer seem fine, given what your father has to do with it.
BYOD is getting more and more common for tablets and phones. But it's new to me that it's used for printers.
Finally, be sure to educate your father about (a) backup and (b) safely using Internet.
I need help with buying my dad a laptop. He is an elderly man and is unfamiliar with and intimidated by computers. He will be out in the "field" selling life insurance policies and inputting the information on the computer, then uploading it to the company headquarters for approval. It needs to have a strong battery and adequate power to upload the insurance policy from out in the middle of nowhere. It would need to be wireless and very secure. I would like for it to have 3 or 4 USB ports. He will also need a small printer that he can take in the car with him (nothing large). Would someone please help?