Ufone didn't have any unlimited internet pacakges last I called their help line, but it could be that the guy didn't understand what I was saying, so its best to confirm again.
Zong works absolutely ridicuolous in some areas, and Okay-ish in others. I used to get 2KBps in Islamabad on EDGE, and around 8-20KBps in Lahore.
Warid's speed was consistent whenever and wherever I used it. 5KBps when it was GPRS, then 20-26KBps on EDGE in lahore after July 2008. But I don't think they have upgraded the rest of the country with EDGE coverage yet. Though you should confrim from their help line.
And you can check if you have an EDGE coverage if an "E" appears on your mobile phone, somewhere near the battery and network signals bar.
But before buying a postpaid connection of any of the companies, you should check their speed in your area on prepaid connection first. Though keep a limited balance in it, as it will drain very quickly.
As for security, you should be ok as long as you are using a nice Firewall and know your way around Internet. You will get lots of Netbios attempts though as they use a private network with a static IP (Though last I was using Warid, their internet traffic was spread between 8 servers each time a request was made).