Sorry to see you selling mate.
I reckon you'll get 10k, but only if you split it. Being an auto too will mean its work about £750 - £1000 less.
I got £8k for my JDM Auto with 62k miles on it last May, with most mods removed and returned as stock as possible. However with the current market, i would say now i would get closer to £6k now...
If you budget that your car is worth £7k, which is a push for an '03 JDM Auto, then make what you can from the mods
Very roughly off the top of my head:
JP Vizage Type N Bumper - £300
JP Vizage A Skirts - £250
Ings +1 Style Rear Bumper - £250
Nismo V 1 Spoiler - Probably leave this on...
Dream Customz Kuro Head Lights - £100 + swap for UK pair
19 inch Fuel Black Deep Dish Alloys – Front 8, Rear 10 - £750
MW Tech Front 20mm Spacer and Rear 15mm Spacers - £140
JWT Pop Charger - £75
Polished Plenum - £80 + swap
Rare JDM Harness - £100
Powerflow Custom true dual exhaust - £300 + swap
Summit Strut Brace - £150
Carbon Brake Fluid Cover - £35
Carbon Battery Cover - £35
Carbon External Surrounds - £75
Custom Induction Pipe - £30
Samco Purple Hoses. - £50
Zed Oil Cap - £10 + swap
Thats £2700 right there... then factor in another £100 to get another set of 17" JDM's if you dont have yours anymore...