[nobody7290]

I dont know if I understand this correct:
"Full advantage":
If you want all options, then yes you need a BES 5.0
But, nearly all options:
BPS 4.1 (this is also a BES, however, free for one user, you have to buy CALs for the others).
If you are fine with the email only sync, stay with BIS.

I dont think, you need a real admin. If you have someone who does the setup of exchange, he maybe is able to setup the BPS/BES.
The BES runs nearly without any regular maintenance. If you have someone in the company, who is able to setup a new email/user in exchange, he should be able to activate a new blackberry also.

If have several clients who also have a small scale BES server (<5 users). I never had a request regarding a problem if the BES. To add a new blackberry, You need the blackberry in the customers hand, and remote access to the Server. Takes 5minutes to activate the new device.