Dear K.,
Here is my humble oppinion regarding your question:
1) usually you can have speeds ranging 64-768Kb/s.
2) let's mention just few providers I can think of:
www.verat.net
www.maksnet.net
www.scnet.yu
www.yubc.net
www.tehnicom.net
3) Signal depends on many factors (how close to the AP
you are, quality of the equipment (especially antenna)
you are using, are there too many wireless users near
you) - the best thing is to have some guys from provider's
technical support to come to your appartment with
their equipment so that they can measure signal strength!
4) Basically, you need antenna, wireless card and a cable
to connect them. You can have antenna's for as cheap
as 30 euros up to 100-200 euros. On the other hand,
wireless receiver/transmiter can be simple PCI card
or external hardware which may include router or something
else you need.
Also if you need to put antenna on the roof you need to
buy external AP since there are some limitations regarding
length of the cable to connect antenna and wireless card.
For more info regarding where to buy equipment please
take a look in the:
http://www.elitesecurity.org/t29170-Vazno-Nabavka-wireless-opreme
Wish you luck,
D.