sorry for late reply .. had'nt read this post for a few days, to answer your questions....
1:- im with ORANGE
2 :- I have a 5 gig limit for £30pcm ( i find that more then enough for the normal surfing, youtube watching etc, if you go over your limit, the rate is pretty reasonable largest bill i had was for £39 and i was 3 gig over the limit ( left it on over night on a download ) the speed of download varies to the signal i average get 7.2mbs (you can setup warnings at 80% of limit reached and again at 90%, you can also set the dongle to stop all connections at the full limit reached so not to cost you more untill the bill is paid but its up to you i just use the 90% warning as a guide im going over.
3:- network coverage is awsome i have found no where i can't get a signal, the download speed is slower then the normal landline connection but thats due to the GSM signal it runs off but after a few days you get used to it and honestly its good enough for normal net use, i have used the dongle on the train, in a car, i even took the laptop fishing on a beach where my mobile can't get a decent signal but this thing still managed to pick one up.
check the orange website for the latest deals, but do not go with 3 !!! there a ********* nigthmare to deal with!
orange gives you the dongle & laptop, laptop has a 1 year warrenty as well dongle is self installing plug and play littraly 1 click start up my 5 year old daugher can start up the net in the mornings to play on the cbeebies website its that easy