I used plain Debian with Samba installed to join the machine to the Windows domain and provide a network share for Windows workstations. I don't knwo exactly what Apple's Time Machine does, feature-wise, but I understand one of the main headline features is that it stores all previous versions of your files, stretching back over time. This is actually quite easy to do, you just have a script de-duplicate the filesystem after doing a backup with rsync or whatever.
Other then that just a case of software which there is open filer or free nas - any thoughts on this as I was hoping to use mine in conjunction or with time machine for my apple mac, intend to use it for more then one apple mac in the near future.
This is exactly why I prefer mdadm RAID, it allows much more flexibility in re-configuring arrays and messing around adding more disks on. Assuming you're adding a number of disks at a time, you could use Linux's Logical Volume Management on top of whatever RAID implementation (hardware, some driver, mdadm) you use to combine RAID arrays together - each time you add a new set of disks, create a new, separate RAID array with them and get LVM to combine them in to the pool. Alternativly, see Ceph, above, which might be able to do the same trick with local volumes as well as network ones.
To start with I was going to get 10 drives and then add more drives but not sure how I can dynamically add more storage and increase capacity if thats possible, seeing as the drives would be attached to the raid card would I have to erase everything and re do the raid config in order to use all the drives or what exactly?