For the former, I know there are guides out there, I just can't seem to find any right now. But I'll try to summarize below:
You ought to be able to install mzscheme with just
apt-get install mzscheme
although you may have to be careful about which version you get. Version 352 is the recommended, although 372 should work (at least with Anarki). Anything newer will require modifying the Arc source, and is not recommended.
You can start the server in a thread so that you can still access the REPL afterwards.
> (= app (thread (asv))) ; or (nsv), as the case may be
This should run Arc on port 8080. To serve port 80, I suggest using Apache. (http://arclanguage.com/item?id=3498 is an example of an Apache configuration for Arc.)