He couldn't use Mokuton well because, as Tobi said, he couldn't control Hashirama's power. He needed Hashirama's powers to obtain Izanagi and used Orochimaru's experiments to length the time of the jutsu.He couldnt really use mokuton, otherwise he would easily break sasuke's susanoo with it,Hashi beat Madara with mokuton...so I think he wanted the 1st's cells not for mokuton, cause he knew he wont be able to control it well, but because hashirama had a strong lifeforce and he wanted to prolong izanagi's time as long as he could, but IMO he could use izanagi without senju dna too, but then it would have lasted only for seconds ...
It is heavily hinted he got Hashirama mainly for controlling Kurama. Hashirama had a natural ability to tame Bijuu, further Danzo had the Sharingan.
If you assume Danzo had Kotoamatsukami, then he also had MS: cementing the idea that he wanted to control Kurama. Thus taking Hashirama's DNA over other Senju (such as Tsunade or Tobirama) makes sense.