It’s hard for a collector to value this. Most collectors simply wouldn’t buy it but would probably say around £300-400 max. However, as you say, a lay person who wants a vintage dress Seamaster with a black dial (which are much less common than their silver dial counterparts) would probably pay more. Black redials do sell for what I consider stupid money, up to and over £1000, especially for ‘Don Draper’ devilles.
Personally, I wouldn’t buy it at all, if I had to stick a value on it I’d say £300 but wouldn’t be surprised to see it sell on eBay for £500, maybe more.
I would strongly advise patience and either buy an original silver dial or settle in, do some research, keep an eye on the private sales forum here and buy an original black dial which will be much more satisfying.