Best practice when purging records?

Best practice when purging records?

I will need to purge records from my 3 hosted databases to keep below the 100K usage limit.

Are there any tips to making this go smoothly?

For example, can the database be live (i.e. available for users to input new records) while simultaneously deleting records?

If not, how do I take a data base "off line" temporarily?

Assuming I want to delete 10000 records from each data base, approximately how long would that take?

Should I work on one data base at a time, or can I do a batch delete on each at the same time? 

Should the database be backed up BEFORE attempting a "Bulk Delete"?

Any tips would be helpful, I don't want to create a problem.