Subframe is probably less than 50 pounds but it is not so much the weight as it is the size of the darn thing. While I have managed to get the subframe back into place by myself a few times I really recommend having some help. Will you be dropping it out with the lower control arms and the sway bar still connected to it? If you have everything disconnected it will indeed drop when you shoot the 4 bolts out, I recommend having a couple of jack stands handy to drop it onto. For the 850 and later cars I recommend having the Volvo Vadis disk for a decent repair manual, otherwise I hear that the Bentley is decent but I have never cracked one open.

Mark