There used to be a set of Tanith on GW's website. Dont know what happened to the "Collectors" section under the IG army tab, but that's where it was. Got Gaunt and 4 or 5 Ghosts. It, IMO, is possible, but expensive if you wanted to get just the Ghosts. I'm talking about the set of Ghosts here. And, besides, you'd have the same 5-6 guys over and over and over in your army and it would look (again, IMO) stupid. Could use some Catachan Snipers, or greenstuff up your own troops. Wouldnt be too high of a difficulty to make your army.
Now, the important part. Every list I see on here has Mech in it. Ghosts dont use mech. So, you'd have to have them "combined" fluff wise with another IG unit to take the field. For example, if you say that the 123rd Terran Armor Regiment (yes I just made that up lol) was deploying in the field with the Ghosts to give them mech support, then there you go. Now, you got tanks! It's possible to put together a Ghost army of all infantry. But from the lists I have been watching, I dont think it'd be all that effective. So if your gonna do it, think about having them "field combined" with some other IG units. And that's only if you want it to match fluff wise.