For plowing, loader weight is not all "good weight". The weight in front of the front axle will reduce the weight on the rear axle due to the "teeter-totter" effect of leverage. It's not a big deal on MFWD machines, but shows up readily with 2wd.
If you have time and a few bucks, you can make cement wheel weights. Many tractor wheel centers have holes for bolt on weights. You can use long lag bolts, some plastic sheeting, plywood and create a cement weight that will add quite a lot of weight. You will be smart to include a couple hooks to allow the removal and replacement of the weights with a hoist, chain fall, or loader.
On the trade, the older machine with 7700 hours is getting pretty long in the tooth. Depending on prior care, it could be waiting to fall apart or still run strong for 8000 more hours. The cost of extra weight on a known GOOD machine vs getting an unknown machine with lots of hours needs to be evaluated carefully. personally, I'd keep the "bird in hand".