i would say forget the dongle idea, but if you must use 1 then make sure it is three network
1, you will need to get a GSM router that supports the dongle so you can plug into
2, o2 and other networks use internal ip adresses which will make it near impossable to get your dydnsor no-ip host name updated
3, response times on mobile networks is poor. the best i could get was around 150ms using hspda and that is living 50 meters away from the mast
4, all mobile networks dont use hspda all the time only when you start downloading something it will jump from gprs to hspda and on gprs response times were even slower around 300-400ms. there is a work around but will require a constant ping to a web site to keeps hspda mode alive
5, altho it is a nightmare and not very easy to set up it does work ok for most providers, uk sly nds was a serious problem cause they require very quick response times. breaks and freezes are to be expected