Joining your client computer(s) to the domain won't take away the ability for you to continue logging-on as the local user account you're already using. Group Policy will begin to apply to the computer once it's joined to the domain, but in a stock Windows Server 2008 Active Directory there really won't be any Group Policy settings of consequence applied, so for all intents and purposes the client computer will appear unaffected by joining the domain.
With respect to your concerns re: applicaitons and setttings: Your existing local user account's profile can be migrated to the domain user account's profile, and this will preserve most settings. One item that is particular offensive, however, is applications that store paths referencing "C:\Documents and Settings..." (or "C:\Users..." if you're in a Windows Vista / 7 world) in the registry. In this case, if you want these paths to continue to "work" you must perform the profile migration in such a way that the domain account ends up using the same directory on the local hard disk drive for profile storage as the local account, which ultimately means relocating or deleting the local user profile.
Personally, if I were you, I'd join the client comptuer to the domain, logon once with your domain account, reboot (to be sure that there aren't any open handles to any user registries left open), logon as "Administrator" (assuming the local account you've been using isn't "Administrator"), and use the "Copy To" functionality in the "User Profiles Settings" dialog, reachable through the "Advanced" tab of the "Properties" of "My Computer" to copy the local user account's profile over the domain account's profile. Specify the domain account in the "Permitted to use" section of the "Copy To" dialog.
I'd logon as the domain account (which should then have the look-and-feel of your old local user account), and use REGEDIT to scan thru HKEY_CURRENT_USER looking for references to "C:\Documents and Settings\old-local-user..." and alter them to refer to the new domain account's profile location. It's tedious, but you'll probably have some stupid applications that saved absolute paths to profile locations. (Thanks, stupid app developers...)
The nice thing about this method is that it preserves the local user account. You can still logon as the local user if you find that things aren't working with the domain user account.
The bad thing about this method is that it preserves the local user account. You can still logon as the local user, and in doing so, make modifications to locally stored documents, etc, and end up getting confused about what's what and destroying data.
As soon as the domain user account is working as you'd like it, I'd delete the local user account and its profile. If you're paranoid, back-up the profile directory and just disable the local user account. Personally, I'd try to make a clean break so that you're not kicking that old user profile directory down the beach forever.
Finally, if you don't have access to someone with good Active Directory expertise re: designing your Active Directory, using Group Policy to deploy software / centrally control update deployment / centrally store user data files / leverage romaing user profiles, etc, I'd encourage you to look for somebody who can spend a couple of hours helping you out. You can get a lot of really neat functionality out of Active Directory, and some very simple Group Policy settings can be used to get user data centrally stored and backed up, get Windows and Microsoft application updates being centrally deployed and managed, etc. It's good stuff, and can save you a lot of time and headaches down the road.
Good luck!
Best Answer
I don't see having Active Directory (AD) as adding complexity. Rather, I see it as making administration easier. I see the functionality that it enables in the client OS as being a major tool to allow for smooth future growth and replacement of computers.
From a cost perspective, there are very low cost versions of Windows Server (2012 R2 Essentials currently fills this niche) that bring a lot of nice tools to bear on small networks for not a lot of money. For small environments you don't have to mess around w/ CALs, too.
Speaking about this in a "bigger picture" view, where Active Directory is just part of a feature set that a dedicated server computer and server OS can provide, I see a lot of advantages.
Active Directory gets you single-sign-on, Group Policy, and the ability to create authorization schemes using Security Groups that will easily transcend employee turnover. In small businesses, in particular, a good permission strategy revolving around AD groups assigned to employee roles has enabled me to easily handle "Bob now does John's job" type situations (which seem to crop up more frequently in small businesses than large, in my experience) very easily.
Having WSUS is great. Oh, boy, I like having WSUS.
Did I mention Group Policy? Folder Redirection? Roaming user profiles? Oh, how I love stateless (or nearly so) client computers and the ease with which I can factory-reload a failed PC or replace a computer. Having users able to logon to any client PC and have basic functionality (client-side apps non-withstanding) turns "drop everything" emergencies into mundane service calls.
I like having a "real" server to handle infrastructure protocols like DHCP and DNS (versus some wonky toy "servers" built into a consumer-grade Wi-Fi router, etc).
Security auditing is much, much easier in an environment where centralized authentication and authorization are present.
I'm a bit partial to the PC backup functionality in Windows Server 2012 Essentials for very small Customers where otherwise getting them to spring for a couple spare PCs to be used in a "hot desk" capacity in the event of PC failure is too much for them to spend. It's kinda hokey, and I would prefer not to backup anything on client computers at all, but the time savings in small shops where client computer standardization is nonexistent is hard to argue.
The business might get value out of other bundled applications that the server could host like, say, SharePoint.
Offering users remote access through Routing and Remote Acess Services or Remote Desktop Gateway.
I like having an on-premise Windows Server with Active Directory in environments where there are Windows client computers. It makes my life easier and ends up costing my Customer less money, in the long run, than trying to "herd cats" by managing a fleet of non-domain-joined PCs.