Pulling the drain plug on the transmission pan will drain out 2 - 3 quarts. Refill with two quarts and top off until full. Usually 2.5 qt. Pulling the pan and then pulling the filter can drain out about another 1 - 2 quarts.
Toyota's filter on those transmissions was a screen and if the fluid was changed on schedule there really wasn't a need to replace it. However if the car has seen infrequent ATF changes then the screen may have begun to get a build up on it and perhaps restrict the flow somewhat. It's easy enough to drain and pull the pan and check to see how it looks.
As far as future servicing goes, if the trans is functioning properly, and it's original, at 186,000, then it probably was taken care of and I would continue servicing it on schedule. In your case I'd plan to change it at 200,000. If you assume that it's going to go out soon and ignore proper servicing then it will go out. Those transmissions were very good and if serviced properly, and the car wasn't hot-rodded, they can last quite a long time. 250,000 isn't unusual. I knew of one that was pushing 300,000.
As far as changing out all the fluid now, I don't think I would. If you don't trust the previous owner, that it was serviced at 176,000, or don't trust the shop he used, then you need to reconsider.
I always like to use ATF that the factory recommends, but in this case there is no telling what all is in that trans, so a MaxLife may be OK. Ask Brake Masters what they used - it has 10,000 since it was serviced there and it's still going.