Email Archiving -- inactive accounts ?

I have 100 old PST
archives of ex-employees

What under $1,000 onetime onPremise
email archive solution do you recommend
that allows the below steps, using a regular
Windows 10 Pro machine with 24" monitor,
not a mobile device ?
  1. MGMT says they need access to 10 employee
     accounts to do a search on something
  2. I give MGMT a login to the Archive Software
     instead of an RDP session to a computer with
     all 10 PST files attached to it
  3. MGMT does one search that searches
     all 10 mailboxes at once
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Below are what I looked at so far based on 100 email users

  1. http://www.mailstore.com/en/mailstore-server.aspx
       ** onetime $2,000 --> use forever

  2. https://www.barracuda.com/products/messagearchiver/models/1
       ** onetime $4,000 --> use for 5 years

  3. http://www.gfi.com/products-and-solutions/email-and-messaging-solutions/gfi-mailarchiver
       ** onetime $??? --> use for ???? years

  4. http://help.smartertools.com/SmarterMail/v6/Default.aspx?qq=%2fsmartermail%2fv6%2ftopics%2fsystemadmin%2fsettings%2frouting%2fmessagearchiving.aspx
       ** onetime $??? --> use for ???? years