That should be fairly easy to spec...
Active Directory for 100-150 users (including standard networking services like DNS, DHCP, etc.) will not use much in the way of processing / ram, so the server doesn't need to be highly spec'd.
The main things I would recommend looking for are;
- Raid 5 as a minimum (RAID 10 preferred) - something like a Smart Array P420 controller
- Hot swap HDDs (so if 1 fails, you can replace without turning the server off)
One of my branch offices has this server
http://h10010.www1.hp.com/wwpc/uk/en...961.html?dnr=1 which should be comparable to what you need.