2600xm's ideally, or large enough 1700;s, FXS, FXO cards, ethernet interfaces, an ip phone or two - ( Handy if you can incorporate a palmtop with softphone, wireless laptop with softphone, voip handsfree handset, old analog phone ). If you can get callmanager and run it on a spare machine all the better. I used callmanager express.
Basically it ain't going to be cheap but it will hold its value well on ebay. You need IOS and tcl files etc to upload and that isn't easy unless you have the access to these resources. I was working in distribution when I did the course and we had a lot of demo kit, so was lucky, but its great fun.
I am sure there are ways of doing it cheaper but if you are new to voice you have to get hands on, it is just too broad to learn it all from books.