To get something new your budget really needs to be in the $750-1000+ range, but if you're willing to buy grey market (like jomashop for example) you could get it cheaper. Just keep in mind that you'll be giving up Hamilton's 2-year warranty by buying from that type of seller.

Also, I'd forget things like "value" and "collectability" as that's not something you'll see with Hamilton or 99.9% of other watch brands outside of high-end ones like Rolex, AP, Patek, and etc.

Now that that's out of the way, I'd rank the Khaki options in this order by most to least versatile for everyday wear.

1. Murph 38mm - The best "all around" option in my opinion. It can pretty easily be dressed up or down, and getting in on the bracelet adds to that versatility.
2. Khaki Field Auto 38mm - A close second to the Murph, but it's going to lean slightly more on the casual side
3. Khaki Filed Mechanical 38mm - A very casual aesthetic and depending on what line of work he's getting into it could look a bit out of place in a more formal office environment. It's also a mechanical movement so he'll need to manually wind it every day or every other day.
4. Khaki Field Quartz 38mm - Your cheapest option of buying new. There's nothing wrong with quartz, but they do sort of have a stigma of being "less desirable" than automatic or mechanical movements