Problem with Gooba is he isn't a great tap ruckman from what I've seen. Not sure he'd complement Marshall near as much as Ryder, but he'd be handy back up for Marshall.
Of course not, if there were a heap of Ryders out there every club would love one. Darcy might be the best of the established rucks available as StC said.
The second tier rucks seem to always be a good option. Pittonet, Lycett, Hickey, Nank types were all second rucks at other clubs that have moved and been revelatory.
Goober is probably more a second Marshall type who will score when he's up forward but probably not going to be a huge asset as a true tap ruckman. That said he's still vey young and inexperienced and would still be a development project. Max Gawn really started to come on in his mid 20s, before that he was very average.