Orochimaru's interest pertaining to currents events being; the world is currently at war facing the One of the most notible shinobi of all time along with the Juubi, the per-historic Proginator of all that exists which once terrorized the world. To add to that, Mugen Tsukuyomi is only a short time away from completion. With such events, one can only wonder why he would say such a thing:
To understand his resolve, we need recollect his goal:
My theory is; Orochimaru sees potential mixture available to him that would succeed both his limitations and Mugen Tsukuyomi. To understand the mixture we have to follow up certain events.
Orochimaru allocates Senju DNA  (not disregarding Tobirama's DNA also) and conducts test on 60 children as well as Danzo to find the perfect Mokuton-DNA Connection but fails, as the mokuton was never completely stabilized. Yamato's survival was out of sheer luck abd getting his hands on his lost long subject, he would've extracted his DNA for the sole purpose of injecting it into himself giving him Shodai's abilities which he "Long coveted" , it was not only his abilities he seek'd but it was apart of the mixture.
Danzo as head of Root was incharge of clearing the dead uchiha bodies from the Masaacre; which explains his rigged arm. Kabuto's explains orochimaru's goal via a hypothesis which was stirred up while experimenting on Danzo's arm:
Being a man of principle, Orochimaru's reason for seeking the rinnegan (besides power) is explained:
Not to mention that he had long since devised a way of retrieving what was his . Now that he has fused with Zetsu, finally achiving that perfect Mokuton-DNA combination the only thing left for him to attain the rinnegan is:
Even given the above it still does not account as to how Orochimaru plans to counter or deal Mugen-tsukyomi when it arrives. Sasuke's eyes are the key.
Though being a blood relative and a uchiha, it was no doubt an extraordinary feat that sasuke was able to not only break Itachi Tsukuyomi but suffered no exhaustion nor real-life side effects from under going the genjutsu. There is a possibility that EMS may have heighten such abilities to the point that it is able to break genjutsu such as Mugen Tsukyomi. It is a fact that the Genjutu it self can only be acquired if the wielder has unlocked EMS (madara). Mugen Tsukyomi may be an advanced form of Tsukyomi which only needs an inextinguishable chakra source (the juubi) to keep it running forever. Thus far orochimaru is only lacking Sasuke's body; specifically his sharingan to complete his goal.
What about Madara's corpse? how did kabuto get it?
Using the map as a guide, The Second Hokage died during The first Shinobi war which took place Right between Kumogakure and Konohagakure. Based on the Terrain of when the event happened: Couldn't have taken place in the Land of frost or the land of hot water But it would be more applicable to say Tobirama died in the area where Orochimaru formed his "Lab-Village" Otagakure  That would explain how Orochimaru got Tobirama's corpse int he first place. Another example of significance is The village of the hidden grass Kusagakure (pertaining to the portion on Madara). This is where Obito got crushed during the Third shinobi war. Where konohagakure were fighting against Kumogakure also fought against Kusagakure. Also the Location of Madara's Hideout; orochimaru most likely knew of the hideout's location long before kabuto being that he was conducting major experiments in that area. Its the location of the Tenchi bridge where Kabuto met pseudo Sasori and Orochimaru fought Naruto.
-> Activating the Mokuton
The Mokuton Madara implanted into his chest wound which he stole from Hashriama was still nonreactive to the point of his death. Kabuto was Orochimaru's right hand man for years and was adept with his research information and some jutsu; especially mokuton as kabuto was able to identify the Zetsu spores implanted into sasuke also to the extents of knowing Mokuton's weakness:
The Kage Summit stressed on Jugo's bodily construction; explaining it to have similar biological function as Hashirama absorbing Natural energy. To activate it however, Kabuto must have used (not implanted) the cursed seal on the mokuton on madara's Chest; activating it as seen done here.
-> Edo-Madara's "Youthful appearance"
Orochimaru possessed jutsu to preserve/prohibit the deterioration of corpses...or possibly reverse? It's possible, and giving Mokuton's ability to amplify jutsu (Obito's Kamui, Danzo's Izanagi); such a jutsu may have been used to reverse the age of the corpse.