need help on a bruder dump truck rolling chassis

[SuperDutyScaler]

my five year old son has been bugging me for almost a year now after i bought my king hauler to get his own truck. So i just found someone whos trying to sell a rolling chassis that has a front king axle and the rear drives, and a bronze bearing transmission no motor and it has a bruder mack dump truck body on it not mounted. Since christmas is a few months away i figure i can get it now for the right price of course, get a cheap motor and esc somewhere, try to find a cheap used 4channel remote and get it together by Christmas day for him. So what should i offer the guy without makeing him mad? He said no lowballers because "we" know how much these trucks are worth?