Overall, there is no need for a cluster IMHO. If you need HA at the DR site, then do it - but if you don't, I don't see a purpose for it as the stand alone will work just fine. The reason I say this, is that they will be two distinct clusters which makes it a little hard to justify.
Tech person from this cloud company said that single node cluster is
complicated solution and is prone to all kinds of problems like "split
brain", that it can stuck all of a sudden and they have seen so many
issues with single cluster nodes.
So I don't know if the tech person didn't understand, or what, but you can't split brain with a single node... so, I don't know why (s)he thought that, but that's incorrect. There really aren't any issues with a single node cluster other than it's a single node cluster which defeats the point of having a cluster TBH. It's a different story if it'll eventually be built out, but if you're going to wait for a disaster to do so... you're already behind the ball.
Keeping just a single instance, not in a cluster would be faster to replicate as well should something happen.
In the end it's up to you, unless I had a much more compelling reason I'd go with just a single server without clustering. Also, don't listen to their techs anymore ;)
Be careful about decreasing the amount of free disk space. The recommendation for cassandra 2.1 is to maintain 50-80% free space. You can read about it on the Datastax site. The free space is needed during compaction as the SStables will be streamed to disk. Depending on your compaction strategy, you can determine your disk capacity.
Best Answer
As per Datastax documentation,
So, it is clear that topology changes (adding/removing nodes) are not recommended when there is a repair process alive in your cluster