Okay, try and post some pics of the rusty bits when you have collected it.
There is no reason you shouldn't do the crossmember, but as James says, it shouldn't cost anything like that.
It is usual to weld on the crossmember and extensions with the body in place, so the holes in the brackets on top of the new crossmember line up on the tub. You should be able to get a crossmember and steel for a hundred quid, and maybe another couple of hundred for welding and re-coating. That will hold it for a while, but if there is rust in the rest of the chassis, you will need to repair that in due course.