I don't recall when I started to get Divine equipment, but my party got a bunch of them even before getting to the Nameless Isle, at the end of level 17.
In this game, the "right time" to buy equipment would be as soon as you hit a new level. If you play on Tactician anyway. It's probably not that urgent on Classic.
But anyway, you can get A LOT of gold on the Nameless Isle, so don't worry too much about it. I went from ~8k when I first got to the Nameless Isle, to ~198k when I left. And that was without any sort of exploits whatsoever. Just go ahead and buy whatever you need.
Once you get to Arx, the main weapon/armor traders would be Tarquin, Almira, and the few around the market area. Rings/amulets are also sold by a bunch other odd traders scattered around the city. And that's it.
The real question here is not "who sells the best stuff", but "who sells WHAT kinds of stuff". Because they always sell stuff at your level. Pretty obvious.
