Depends but a good gauge to go by is figuring out how many oz of yarn you need (i.e. if using worsted, how many oz, etc) then buy that many oz of roving. Also keep a strand of worsted near you so you can make sure you are spinning a similar thickness. It is an ambitious project on a spindle, though. Currently I only have a spindle and it takes a LONG time to spin.
Also, make sure you have sandbags or something if you get a Babe, most people have trouble with them "jumping" or scooting because they are so light. Weigh down the legs, though, and they are good to go!
The Ashford Kiwi is only $285 unfinished. That's only about $100 more than the double treadle Babe and you get a much better wheel (and it isn't so ugly lol). I am pipe dreaming as we don't have wheel money but once I started looking at prices, $100 did not seem that much in comparison to other wheels out there! After I typed it I realized how silly it sounded to say "only $100"