£50-100 if no engine and boiler i would think is a fair figure to start haggling within. Any more then £100 and you ''could'' build a new one from plans at the cost of material, the photo could be deceiving it may look far nicer in the flesh then the photo makes out which then you can allow a bit more.
Best way to haggle the price though is to ask ' how much do you want for it' when he comes up with a figure knock 30% off it, (so asked 100 offer 70) and offer him that, hence the haggle! be sure to point out damage, chipped paint or varnish etc etc as this helps you haggle the price low as you have to repair the model at cost!
But if its awful then just walk away and take the cost of the trip on the chin, last thing you want is to spend and spend and it never looks nice.