if you are set on pruning..
Due to the large number of pruning that it appears. Not knowing how large it currently is.
You probably will need to run the appropriate rebuilders in the ubb control panel when you are finished pruning..
As far as a backup site at one.com
what is the point to have the complete old database there.
You can't export the pruned ones.
So you still would have the cost of a second site
Try calling
https://expresshosting.net by phone and ask Tom.
On his plans he is more concerned about bandwidth and total storage than just database size.
Tell him what your needs are.
Be prepared to answer questions, he knows ubb and it's requirements very well because he hosts several members here..