300 pounds is rather low even for 2nd hand.
The RME cards do have a mixing application within it (i belive its called total mix) and they come with a pci card and you may also get a cardbus which allows you to connect it to your laptop and go portable.
having said that the Muto 828mk2 is very simmiler and features about the same quality of convertors&flexibility.
not sure if you can find either for less then 500-600 euro on the 2nd hand market.
http://cgi.ebay.com/RME-HAMMERFALL-M...cmdZViewIt em