The Smithy is not a bad machine,but it is a 3 in 1,so it's a compromise.You can do all the milling you need for crossbow work in a lathe.I would suggest you read up on lathe work or watch some videos,better yet,subscribe to some magazines,there are 3 good ones,Model engineer,Model engineer's workshop,both of these are British publications,Home shop machinist is American.In order to mill on a lathe,you will need a Vertical slide,which is fitted on the lathe on the cross slide.The best lathe for this,is a Myford,as it has a "T" slotted cross slide,a good used one is ideal.Another lathe that is suitable is the American Atlas lathe,you can get milling attachments for it. There are quite a few good sites on the internet that describe and show machining operations on the lathe.
Lathe sizes:there are 2 dimensions that are used to define a lathe's size,one is "Swing",this is the distance from the centre of the lathe's spindle,to the top of the lathe bed,or ways.Most lathes of a size suitable to crossbow work will be either 7ins. or 9 ins. swing.double this measurement and it will give you the largest size of work you do on the lathe in theory,(practically,it works out to a bit less)the next dimensions will; be"between centres",this is the longest piece of work you can put in the lathe,a lathe suitable to you would be 20-24ins. between centres.As you can see,this is a HUGE subject.It would take a few hours of conversation to tell you all you need to know.My best advice to you,would be to find a really knowledgeable Model Engineer,or a skilled machinist,in that order,read books on model engineering,and watch videos on You tube,there are a lot out there.As I have said,you can do all the milling you need in crossbow making on a lathe,if you have the right kind of lathe and a top slide,in any material you want,steel,aluminium and plastics,a lathe is the first machine you should go for,but you really need knowledgeable advice from a machinist before you buy,particularly if you are on a limited budget.Any other questions you have,I would be pleased to answer,so if you have any P.M. me.With machining,you need the best advice you can get.otherwise you can spend a lot of money on machines that will dissapoint you,because they are not suitable to do the work you want them to.