More pictures would be a big help if you want an accurate valuation. The picture you have up at the mo make the bonnet look as if the paint is patchy or faded. Pictures of the interior would help too. As a rough guide though, being a GS with that mileage, engine size etc... assumint it is as you say near mint, the starting price would be around £450, possibly a little more if it's a 4-door.
If you want to up the value to sell it then a damn good clean inside and out is a great place to start, nobody wants to pay top dollar for a car that looks neglected. Good tires, long tax & mot also help out.
Colour-coding the bumpers would help promote it to GSX level and would take years off its age looks wise, but you would have to colour-code genuine nissan style and not just simply cover the whole bumper in colour, otherwise it will be obvious that it's not origional.