Bring a standard PC power cord. It fits in eMacs and you can turn them on.
Also a USB keyboard and Mouse wouldn't be a bad idea if the seller isn't including them. Then you can open the About This Mac and open the CD door.
The first 700mhz eMac had a silver bottom plate, while I know the 1.42ghz model had a white plate. The 1.25ghz also had white bottom plates. Not sure about the models in between those. Examples of pics:
700mzh silver bottom,
1.25 white bottom
Check everymac.com's
eMac page to check model numbers and specs.
M9834LL (1.42ghz) and M9425LL (1.25ghz) are the two top of the line models with usb 2.0 and upgraded graphics card and much faster ram (PC2700 compared to PC133 of all of the earlier eMac's). They can also run 2gb max of ram compared to a maximum of 1gb in all of the other models.
I would avoid the 700mhz (M8655LL) and 800mhz (M8892LL+M9150LL) models. They're slower, have slower ram, usb 1.1, etc.
As someone else said, there are two versions of the 1ghz model, as far as I know they both have the same model number, M8950LL, but one is the education version which has usb 2.0. The regular version has usb 1.1.