I think it's too high. It's been a few years since Lelys were made. Buying a used disc mower is always risky because repairs can be expensive in both time and money. $4000 is too close to the price of a new one, I would rather bite the bullet and finance a new one if necessary.
Things to look for on the used one if you decide to get it or make offer on it: Mount it on tractor and listen to it run. Raise cutterbar and look at skid plates. If the plates are worn they can be welded up. Notice that the C bar has cast iron segments for spacers. The skid plates do not protect them and after a lot of use holes wear in them, which can be welded up with some labor. The cutterbar segments are fastened together with a long bolt that goes the length of the bar. If the mower hits something the bolt can be bent and can possibly be be straightened, otherwise replaced. To check and see if bolt is bent, put your foot against blade carrier(engine off of course) and see if the blade carriers can be turned. If there is a lot of resistance, the bolt is bent. There is a square shaft that runs the length of the bar and drivesd the cutterheads. It is run from the gearbox and the shaft can get rounded off at the gearbox where the power is applied. It is a special hard steel and can't be welded up and reground. It is also expensive. The blade carriers get worn by the passage of hay over them and will wear through next to the blade bolt. They can be reversed to equalize wear. They are of hard steel and can't be welded up.
The individual cutter head segments that drive the blade carriers have some sealed bearings which rarely give trouble but have to be checked. This is no problem since the gears there have to be cleaned of old grease and new grease put in so bearings are checked then. It's a special grease someone on here said might be cornhead grease. To get to gears, remove blade carrier by taking off the 1 1/4 or metric equivalent nut and then removing the bolts holding the assembly in. Do this one at a time and reinstall keeping the blade carriers in time.
The main gearbox on the older Lelys is also packed with grease. The late model Vermeer I have uses gear oil and I'm not familiar with all the steps on greasing the Lely gearbox.
Drive belts need to be pretty tight and should be replaced as a set if necessary.
None of what I said should be taken as criticism of Lelys, they are good, dependable machines and don't give trouble if properly maintained.
Hope this helps.