Theres no limit. You wont hurt it--most likely. It really depends on what kind of condition the steering and suspension are in...as to how safe it is to drive it at a higher rate of speed. The bushings (C-bushings, rear radius arm bushings and trackbar bushings) can greatly impact the ability of the vehicle to track straight...in that case it tends to feel like the front end is wandering--alot of olks attribute this to their steering box or shaft--but its usually the bushings...it depends on how bad it is and how brave you are as to what speed you can safely go. Long before I ever upgraded my suspension--the first time--I used to drive my Bronco all over the southeast from Mobile, AL...Orlando, NOLA, N. Carolina, Atlanta, Tuscaloosa...wherever, whenever sometimes quite fast...you just have to get used to it. It could be that yours is in good condition--steering-wise.
If you have some tools or a place to do the service when you pick up the Bronco, you may want change the oil and some of the fluids (drain and refill radiator, tranny, transfercase) and check the oil levels in the differentials. At the very least, check all of these levels and pick up a couple of quarts of oil, a gallon or two of coolant/water, and some gear oil. You could always take it to a shop...but it all depends on whats in your budget.
Be aware of all of these things, but try to put them to the back corner of your mind and enjoy....some of the best times and most beautiful drives have been road-trips in my Bronco. Welcome to the forum.
If the oil is clean, the steering is tight and the mood is there, 70-75 should be achievable...the engine can run all day at 3000 rpm. As well, actual speed may be different than what the speedo reads, if it has larger tires, your speedo will read lower than the speed you are going. Try timing mileage markers with a stop-watch over a few miles to get a good guess-timation. Safe travels and have fun!!