Newbie question

Hi all. I have som questions regarding a new Asterisk installation about to do.
Currently I have an Ericsson 128i PBX with 50 telephone lines. The PBX is connected to a ISDN modem (Alcatel Litespan 1540 NT). Each line has a real phone number which you can call directly from outside.

I want to change this system with an Asterisk but I´m not sure about what card I should buy. After reading some post here and in I think I should buy a PRI card to enable the connection to the ISDN line. Here are my questions:

  • In ebay it´s about 2100 dolars. Can I find something cheaper?

  • I´ve read this cards has 2 channels. Will I be able to perform more than 2 simultaneous extern calls through the ISDN link?

  • Can I use the Ericsson PBX I have? This is connected to a propietary-software windows server through a serial port.

Thanks all.