My main concerns would be bugs and critters if it has not been sealed up good, moisture if sitting on a concrete floor, and if it was in a hot building during the summer, I have seen the rubber seals stick to the metal drums and tear when put into service. What I do is pull the pan looking for dead bugs and rust, then I pull the valvebody and air check while spinning the input shaft, if it air checks good I use them as is. At least in my part of the country, seems like bugs and mud dobbers get in everything. I had one just the other day that had been sitting and the low reverse band apply lever rusted and froze, it would not move by hand anymore. Maybe the servo would have broken it loose after fluid got in it, but I took apart and freed it up.