Gotchas? Buying a milling/drilling machine to make it CNC?

Me again! :)

I know that no one could give me better advice on this, than you
guys, so here goes.

I'm looking at the idea of getting a bigger mill than my sherline,
but have been considering getting a milling/drilling machine (like
this one http://cgi.ebay.co.uk/ws/eBayISAPI.dll?
ViewItem&item=2598536382&category=633) and upgrading it to CNC. I
have access to a pretty big school workshop, so I should be able to
make any kind of needed shaft-convertors for attaching
steppers/motors to.

Now my question is, "what's the gotchas?" Am I going to be terribly
sorry for even trying this? Will the spindle on this mill/drill
machine wibble too much for CNC? Will it bring me undue suffering?
Any tips?

Thanks guys! (and girls)