haven't read whole thread but here's my take on it:
any broadband connection can be wireless if you have a wi-fi modem and the capability on your computer.
if you're computer isn't wi-fi enabled you can buy the necessary equipment in form of a pcmcia card. belkin are a reasonably priced brand for those.
most ISPs offer wi-fi modems for your connection. this can mean they charge a bit more on their monthly subscription but you are at liberty to buy your own wi-fi modem.
i have a thompson speedtouch 580 and it works fine. was recommended to me by a wi-fi geek friend!
the drawback is that your ISP will probably always try to blame any connection issues on the fact that you're not using their standard issue modem and try to fob you off.
with regard to security, yes you should secure your modem.
firstly, you secure it against anyone else connecting to it. you can then WEP or WPA enable the connection to ensure encryption.
WPA is more secure but not all devices are WPA compatible.
Your modem instructions should be quite straightforward in how to set up WEP/WPA.
This sort of security is not about virus protection or firewalls. It ensures a secure connection making it harder for people to hack your session.
If you are using internet banking or e-commerce type sites you should already be using SSL as provided by that site. Thus WEP/WPA enabling your wi-fi modem isn't strictly necessary but it's best practice.
As for ISPs, take your pick! Whenit comes right down to it they're all a bunch of shysters who over-charge and never offer the level of support you need when the technical bits go tits up!
BT are particularly crap. I've spent hours talking to their tech supp in India who once told me the problem was static in my modem FFS. As it turned out they hadn't provisioned the line!
HTH and good luck!
ps: can you tell I'm an IT Security geek? LOL