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...