Moved to General since you're not selling but asking advice.
That's a lot of mileage, I wouldn't listen to a guy who says "all you have to do is this one tiny thing..." sounds like sales BS to me and that is the kind of rig you drive away and the engine falls out...
If there is nothing wrong, have them provide a warranty... they won't and you can walk away. $10K seems like a lot for an old high mileage van, but I'm sure some others will have input too.
128,000 miles == 205,000 km
128,000 km == 79,000 mi
The actual reading on the dash would be miles, so that changes things (I misread your units originally) 79K miles is not bad. But I'd still be leery of the sales tactic.
Clear as mud, no?