Your current CALs cover your clients for connecting to your SBS for file, print, AD, Exchange etc. You'll need seperate CALs for the TS (and the TS will need web access so it can phone home and verify them - TS licensing is not just a paper exercise, it is enforced). There are both device and user CALs, but you can't mix and match both. I personally feel user CALs are more flexible (one user can have any & many devices, but you always know how many staff you have). Note: I am not a licensing expert though, I deliberately avoid getting involved w/ licensing where I work and stick to the technical side!
You will definitely need a dedicated TS server - think about the workload of your staff on those 25 desktop computers & the apps they are wanting to run. You'll probably need to do some sums in terms of sizing the hardware - and allow room for potential growth, or just new apps, updates and changing requirements. It probably won't be a terribly cheap server. But it is important and definitely not something you want an SBS doing (it's already doing a lot of tasks!)
Another approach regarding your problem (which sounds like mostly trying to save time) of building PCs would be to partially or fully automating this. Check out Windows answer files, Windows 7 has something called WAIK and server side, check out RIS (maybe a deprecated name nowadays - I think it still exists) or SCCM. Also Altiris (aka HP Rapid Deployment Pack) is extremely powerful - we build 2 very complex Citrix (effectively TS underneath..) environments with it (2 different O/S & 100s of apps. It works very well but I don't know how much it costs and is probably overkill for a simpler environment).
My personal answer though, would be to standardise PC builds rather than go down the TS route at this point in time. Automated builds can be very easy to manage with something like Symantec Ghost or some of the Microsoft tools, as long as you are buying a standard set of hardware models (driver management becomes easier this way - and MS SCCM makes this easier, it can add only the necessary drivers for each HW model - no need to make an image for each one). As PCs age, you can simply replace them with more powerful models. PCs can be replaced in batches too depending on their age (i.e. as they get to 5 years old for example) - makes budget planning simpler too, as you can predict what is going to be replaced.
You've asked a fairly broad question so my answer won't have covered all of it, and in fact I've covered managing PCs as well, but perhaps it has helped anyway; it sounds like you've got some build issues (i.e. no automation)
I suspect others may weigh in to help with some extra TS specifics.