I have no idea where to ... find the index.html/index.php which gets loaded.
Look for page source files
One approach is to browse the site to find a more unique page - lets say newcontactform.php - ideally one that is unlikely to appear in other sites hosted by the same server.
You can then try
if that fails, follow by
find / -name newcontactform.php
this should produce a managably small list of candidates.
You can then inspect the files, do diffs and if necessary try small changes (e.g. insert an HTML comment) to verify that the file indeed produces the page.
Find the configs
Sometimes config files are evident in the output of the
ps command. Worst case is
ps -ef | grep -e 'apache|httpd' but more creative use of
ps options might be worth exploring.
You can look for
httpd.conf in the typical locations for Ubuntu and for the Apache httpd project (which may differ) or just use
find as above.
Sometimes the main config file refers to other config files for vhosts. You can work this out by identifying the main config file.
Sometimes, old servers run a variety of webserver daemons. In that case it
can take a while to find them all and work out where their config files are.
A combination of the techniques above should eventually succeed.
You can find what programs are listening on port 80 etc using
netstat -lntp. Often, locating the binaries is a useful pointer to a directory tree that contains the config files.