Is this for a production environment or only a test / lab environment?
Is there a budget assigned to this project?
Does this system generate revenue or otherwise act as a critical component that's breaking would result in negative financial consequences?
Active Directory is a few things at once, primarily DNS and LDAP tightly integrated. SAMBA can only provide for the AD PDC emulator login tickets, it can not provide for the DNS resource queries to the LDAP nor the LDAP service itself. The authorization only services for shared resources not client to client authentication and security roles.
My personal suggestion is if your just learning then sure play with Samba and NT clients, it'll be fun. If this is for a business or production environment then just use a real NT server, it'll be more stable and a TON less headaches. The LDAP and DNS environment is tightly integrated and unless your an expert in ADC you won't be able to replicate those connections and objects inside an open source environment. You can use a Linux server to do lots of things, file services, backup services, web services, application hosting, network security device and so on. The one thing it's really not good for is acting as an AD authentication system for NT clients.
On a side note, you might not even need an AD server. Would RADIUS serve your needs?