mongodb read write locks
I need a process that will be constantly running deleting documents
  older than some expiry data.


This you can do it by creating a Time to live(TTL) on the expiration date. TTL indexes are special single-field indexes that MongoDB can use to automatically remove documents from a collection after a certain amount of time or at a specific clock time. 


  Reading the MongoDB documentation it looks like when I do a write I'll
  be locking out the reads until the writes have completed.


MongoDB wired tiger storage engine gives document level locking. So, in this case, only the documents that are being updated/inserted will be locked. Other documents are available to read.