I have been investigating some of the options as far as the main head spindle goes and there are a fair few out there to choose from. I have not had a chance to get on Solidworks and plan out a rough size but I imagine it will be determined by a component like this anyway so makes sense to decide on the sort of size of spindle I go for and then design around there.

Is there a calculation to determine the overall weight a lathe should be based on the swing over the bed - I dont know why this came to mind but I am sure I read it in my travels.