SATA1- 1.5 Gbit/s
SATA2- 3.0 Gbit/s
eSATA is a cable designed for external drives as the standard cables can detach rather easily and are more fragile.
More detail: nclick here
In theory the devices are reverse compatible (with a loss of performance) but there have been compatibility issues with specific devices.
I wouldn't go there unless I had:
1) SATA2 chipset on the mobo
2) eSATA connector on the case
Sorry I cant recommend an enclosure (I built my own) but these don't do too much with SATA except provide the power but there are multi-function (eg SATA & USB2) which I would avoid.
For cheap backup USB2 drives are hard to beat for cost and convenience but if you want the speed of SATA2 then I suggest buying drive and case from the same supplier provided they will indicate these are compatible