Whereabouts in East Sussex are you? I'm in East Sussex myself and would be quite happy to come along and have a look (not fish!) to see how appealing it looks from an anglers perspective. Might be a long walk from any parking facilities, might be too overgrown/weedy/snaggy to fish???
As far as a possible income is concerned, a lot depends on what inhabits the lake. If there are some right old lumps in there you've got the makings of a much sought after syndicate, which could bring in a decent whack.
If it's just general "run of the mill" stuff, then you'd most probably be better off just renting it to a local angling club as a mixed fishery.
But either way, there would be a fair amount of work required. If it became a syndicate, who would bailiff it for a start? And a syndicate are going to want night access. Is that a definite "no no". Lots of questions remain unanswered.