The difference between an old cast iron Boss and the aluminum frame Boss is that the iron Boss moving parts are made of steel and in the new model they are made of zink castings.
I have had two aluminum Boss machines and both broke down after a while, the first one was defective out of the box, so had to order parts from the US to get them running so I could sell the crap.
Then I found a used cast iron Boss in California, and it has done a great job for years without any problem. That is a difference ! Also I have read about Tippmans excellent customer service here on LW, but my expeience is that it does not reach outside the US . They didn´t even respond my emails! I have replaced my Boss with an Adler 205-374 with a needle positioning system - the real dream Machine !
So my advice is, save some more money and buy a "real" sewing machine from a dealer with a good reputation and custromer service.
My 2 cents ...
/ Knut