I think the best of both worlds mentioned above – affordable, and solid gold, will have to come in the vintage category. One can find fantastic vintage watches in solid, 18k yellow gold for below $5000 very easily. Obviously the Rolex Day-Date is a classic, and one of the most undervalued watches around (if we’re talking 1970s models, in yellow gold). It’s not exactly an understated watch, though.
I think a really interesting option would be a 1950s Zenith chronograph. For example, those with the 156D caliber are just beautiful watches. They perfectly proportioned with a 37-38mm case that is very thin. They are super elegant with white dials, two registers, and they usually have a blue tachy scale, too. Did I mentioned they are solid 18k gold, and you can find them for around $2500 from time to time?