I cant remember all the files off the top of my head for BIND but you'll need to create the zone files for forward and reverse lookup's and then modify the named.conf file in /etc once you've confirmed bind is work thats a start! oh and to use mail you'll need an MX record in your forward lookup zone file.
Now in /etc/mail theres a file for sendmail called sendmail.mc this is the configuration template. do all your changes here then you submit a command with generates the sendmail.cf file which sendmail uses. In the sendmail.mc file you specifiy the the IP's that sendmail listens on i.e 127.0.0.1 and the network IP wether this is a private or public ip doesnt matter but in your case if its only internal mail i guess it'll be within the 192.168 range?
aliases are nicknames to a user account for example... root isnt allowed to recieve email etc for security reasons (i think) so you setup an alias to send all email to firstname.lastname@example.org
so its baically a pointer.
um...you'' find a file in /etc called aliases. edit that. once your done you need to run the newaliases command to rebuild the aliases DB.
i dont see running an internal mail server causing problems. at the moment they only way i see it is to have two accounts setup in your mail client... i guess you can configure sendmail or postfix to send email on to another server but if you havent got your own domain this might be a problem?
in my case i own a domain and use my own mail server for local and external email. if i wanted my isp to relay email for me then i'd have to get them to configure their dns and mail server for me and then tell my mail server to send any external email (anything otherthan my domain) to the isp's server to forward on. but as i said if you havent a real piublic domain this wouldnt work? (i dont think)