I went the same route as yourself and choose to use a 2nd hand SSF PC.
For the simplicity of it your best option is to IBM Netvista. I purchased mine (A 1GHz PIII with 256 MB, 40GB HDD and a CDROM for €100 from ebay and using spares i already had, i upgraded it to 512MB with DVD & a 40GB.
The only failback is the lack of USB2 and the onboard grpahics can only use up to 512KB of RAM making a third party card necessary, saying this i got both a slimline usb2 card with 3 ports and a slimline graphics card for quite cheap off ebay. The machine does come with three pci slots although one will be taken up by the Network Card (GB Ethernet in my case) leaving you two.
All in all though you do end up with good options, mine has the accessories connected in the following sequence:
USB1.1 - Touchscreen
USB1.1 - GPS
USB2.0 - Bluetooth
USB2.0 - 802.11G
USB2.0 - Creative Audigy External (You know the one)
PCI1 - Intel Low Profile 1GB Network Card
PCI2 - Nec Low Profile USB 2 Card
PCI3 - Nvidia MX4000 PCI Card (128MB 128Bit)
For power i am using a Opus 120Watt, apart from drilling some new mounting holes were the original power supply was this install was plug and play. I had now problems at all, and as the org power supply was a 95watt, i was left with plenty of power for my screen etc.
The Compaq's required your to mess with pwer supplies and im sure the e-PC also uses a non standard connector for power. I drive a Peugeot 106 and i have fitted my SSF on it side taking minimum space in the process, far less than a ****tle would have.
If you have any questions then just drop me a pm.