Used prices will vary but I would expect £900 to £1000 for unpowered?
Persoanlly I would go powered if you can afford it, especially if you want to use it live as you can drive anything without having to get an additional power amp.
I also would recommend the remote if you are playing live. I don't know the MC6 but a general midi controller will be good enough but not have all the features, require multiple cables etc. so again if you can afford it, get one but its not essential immediately. As mentioned you might struggle to find a second hand one..