Given the open invite I'm gonna jump right in.
The glass ceiling on prices appears to be at €3k, despite some
delusionalattempts to shatter this ceiling.
To my mind a clean well maintained, '98 manual GPvR, with fresh NCT, no outstanding work and a bit of tax left could expect to achive something approaching 3k probably closer to €2700 in reality.
You have pointed out that the car has some underbody rust which has cuased it to fail the NCT. In reality they all have some underbody rust, but the trigger point is wheather it causes an NCT failure (road safety concern) or not. Having failed on this point the next question is how serious is the problem -is this a replacement chassis leg issue -probably €200 -€300 per leg parts and labour to resolve, or are we looking at significant structural degredation -chassis compromised and flexing =unrepairable write off and unsafe.? In which case the car only has scrap value.
Most punters, myself included couldn't be bothered taking on such a project along with the other NCT failure issues (costs)-just wait until the next sorted car comes along.
Obviously both the NCT and price are important.
The NCT is a prerequisite to an easy sale.
Without NCT you'll have to discount substantially to make this an attractive proposition for someone to take on the project/ risk. ~€1500 IMO.