OWC has a good selection of external enclosures
here. If you can get one using any of the Oxford chipsets, they seem to be very reliable working with Macs (OWC says which enclosures use the Oxford chipsets). You also want to get an enclosure that uses Firewire, so you can boot from your external drive if you need/want to. Can't do that on USB.
For speed, a 7200rpm drive will trump a 5400rpm drive. I don't know of a 120gb 7200rpm 2.5" notebook drive out there today, but tomorrow is of course another story.... If it were me, I'd go with a 7200rpm 100gb drive. Unless you need the storage of a larger drive (which you said you didn't), the 7200rpm drive will make itself felt with crisper response and faster disk throughput.
The newer Seagates have a 5 year warranty, if that's an indicator of potential reliability.