First, realize that both circuits (fed by the forward and the rear reservoirs) go to both front calipers. Each of the front calipers have two independent pistons (which is why Volvo brakes are so safe, viz. redundancy). That is, if you lose one of the two systems, you can still activate three of the four wheels' calipers!
I would bleed both circuits, no matter which one is empty. And I would factor in the possible (probable) cost of replacing all the calipers, if not many of the lines too. A empty (dry) circuit is apt to corrode rapidly (lots of air to introduce moisture), and this can't be fixed by simply bleeding and flushing it. It's a sign that the car hasn't been cared for properly.
I'm not saying that you shouldn't buy it, but it's:
1) an indication that you should look over the car for other neglect, and
2) just a factor (along with anything else you find) to adjust the price that you pay accordingly.
Good luck.