Asterisk is available on the internet for free. There are also some versions with proprietary GUI's and/or other non open source stuff, that folks want money for, so beware of what you are getting. Most periphial cards have brand-specific software that they work best with, although they'll usually work 'OK' with the default Asterisk software.
Can't comment on the Aastra phones, no experience.
Echo can be dealt with, in the Asterisk, either in software or hardware. Need to find out what, if anything, has been implemented. But all of the problems you describe are very often due to poor QOS, either within the LAN, or out on the 'net'. And if they are using a 'dynamic' broadband link....!
If you get out on the internet, there is a world of info on Asterisk, as well as the manual, that isn't too 'dry', so to speak. Don't pay for it, it's a free download. I printed mine out. (What a pain doing 2-sided print with 20 pound paper on a 'economically priced' printer!)
