Hello Roger,
I have an SC4 and find it a very good lathe, but I have yet to come across a milling machine attachment that really measures up against a standard bench mill for any lathe.
I'm sure you will get a great range of suggestions, but one way of saving money might be to look at getting the stand alone base for the SIEG X1 mill head and column fitted to your machine. The X1 was actually quite well though of as a small mill.
I know Arc Euro have discontinued the SX1L mill, but they may have these in stock.
Another option would simply be getting a good X-Y table (new or second hand) or even a broken machine and making a suitable column mounting bracket/adaptor.
Neil