Engine or balance shaft?

I recently acquired a 2006 E350 AWD for $740. The interior and body are in great shape. The car needed a new secondary air pump, an upstream oxygen sensor, and tires. The transfer case (Car shudders on low speed turns) is low on oil or gone. The SBC and cam magnets were done at the dealer a year ago. Everything works on the car aside from the mentioned problems.

It’s been garaged (and dealer maintained) for most of its life and has 78,500 original miles.

My question is should I start saving for a used engine or the balance shaft replacement? Can someone please give me a definitive number on the cost of either the replacement or balance shaft fix? Or should I just dump the car? It looks and drives beautifully at the moment aside from the shudder.

Also would a dealer install a used engine? I’m going to talk to independent shops as well but wanted to see if I could negotiate if need be.