As I said earlier, only pay that if it really is in the condition you think it is. At £9.5K the chassis needs to be immaculate. In practise, that either means it will have had a new one at some time in its history, or the original will have been throughly and repeatedly protected inside and out. Chassis protection could have been done by the owner or they could have paid to have it done. Is there anything in the reciept history suggesting a regular spend on chassis protection?
The problem is that mud gets into the chassis rails and unless it is removed, over time they rot from within. An otherwise rotton chassis can be tarted up by an unscrupulous seller with a tin of waxoyl or underseal so it appears sound at first glance. You need to get on your back underneath it, wrapping the metal with your knuckles or something suitably hard, searching for areas where the metal feels soft or sounds different and looking for evidence of overspray that might suggest a quick touch up has occured, or underspray that might reveal what the true condition of the metal is. Make sure you feel on top of the chassis rails and suspension/body mounts as mud trapped there will rot from the top.
Farm vehicles lead a hard life and from the body work, this is no exception. In the normal course of events its chassis would likely be shot by now, so unless there is documentary evidence of replacement or protection, be suspicious.
Also beware paying extra for a low mileage engine when you know it has only done short, hard working journeys like farm work. It may well have suffered far more actual wear than one that has done twice the mileage, but on longer less stressing journeys.
I guess the best advice is do not buy the first (or even fifth) one you see. Look at a few and you will soon get a feel for what they should look like and what a dodgy one looks like. I would even go to a well respected dealer and look closely at ones you know you have no intention of buying. Nothing gves you a better baseline to judge than knowing what good looks like.