Sanddrag,
I will give you a heck of allot of credit for even thinking about casting your own lathe. The master machinist that taught me how to machine casts engine parts for homemade engines. However the parts require major machining after they cast, not simply a light sanding for clean up. I would be pretty scared to turn a bunch of metal projects on the thing to tell you the truth. Looking at my lathe, the parts are all steel, and they are all machined for precision, and they are all definitely over sized. Even looking at a wood turning lathe that you could go over to sears & buy (Such as this)
http://www.sears.com/sr/product/summ...id=00921712000
looks much more robust that the cast pictures on the site shown.
Looking at the pictures shown, the pieces are the absolute smallest pieces you could use and they don't look like they could hold much of a tolerance without machining them on a mill when you are done. You will definitely learn a bunch about casting in the process of trying to build the thing but also remember that your first pieces aren't going to come out of the molds ready to be mounted on any sort of a precision lathe without some major rework.
Something like this $460
http://www.sherlinedirect.com/merchant.cfm?pid=1&step=4
will probably be more accurate and reliable or you could find one used for even less.
Here is what you can get for even cheaper
http://cgi.ebay.com/ws/eBayISAPI.dll...tegory=13 876 $255
I would strongly suggest you take every precaution for safety when using a homemade lathe or milling machine. They are dangerous enough without having to worry about a part of your machine flying off. Just my $.01 but again kudos for even attempting it.