Hi ken
Dell has new PCs for <$500. The slowest cheapest box they have is overkill for slim only. Dells customer service reputation has suffered recently by farming consumer tech support calls to India. This has improved, but you should keep this in mind. I own and recommend Dells to my business clients because they are quality design and cheap. Farmed out service is part of the reason why the machine is so cheap. You will need to remove all the crap software running on it, like Norton, AOL, photo editor software whatever else. I can help you with that if you just drop it off for a couple days.
Alternatively, if you can find a used or free donator Pentium 3 you could spend far less on this project. Load Linux or minimal XP on it and run slim only. It will still be very fast, especially with linux.
I have Infrant ReadyNAS NV which runs slimserver natively. It is a nice idea, but the processor is too slow to act as host. I don't do any scanning at all, work strictly out of "Music Folder". Browsing is tolerable but noticeably slow. Converting flac to wav at the server for optimum sound quality playing flac is impossible. Running Inguz is impossible. I use my P4 as a separate slimserver for those things and ripping. Infrant is extremely expensive for the advanced features which are nice but unnecessary. Its data throughput is unbelievably fast, good for streaming video and multitrack recording, streaming hi rez wavs, etc. I think for your application a PC is best. Doing it again I would not buy the Infrant, but just another PC. It is noisy too, since a recent firmware update increased the idling fan speed to 2600RPM!! It can't be in the same room as the system. It is annoyingly loud in any room.
You certainly could use your laptop as a slimserver. Eventually you will get tired of squinting to see the SB display, and will enjoy having the slim GUI in front of you. Laptop is ideal for that, but you could still use your laptop to control slim running on another server. If using laptop only you can hang your hard drive on the network, etc. as nas, or as shared drive in another PC that you already own? A free ethernet port on your router will allow NAS. Lots of options. Glad to chat you up about it.
Just think cheap. There is no advantage to spending more money as long as it is reliable. Try to find a donor PC like a Pentium 3 or younger. Even if you find a junker, we can probably get it going for cheap.
Rich