I know you changed your intentions further down and are now saying you hooe you don't have to and are just making preparations in case
BUT
I'll answer the "I have to sell".
You will not get best price at auction. 2003 with mid -high mileage will be a couple of grand, at best.
You will never get the £5k you put into it, back. Unless you got it for free.
Best prices on autotrader range from average vehicle £2.5k to someone's hopeful £5k.
Whilst showing all paperwork is a bonus, it will not make a massive price difference on a 19yr old vehicle.
Sell it as you sell any vehicle, look around at prices, decide how much you will take, add a little for barter room, let the buyer think they did well.
Then say goodbye to it , and look for next love.