I agree with Dave and Pete on this. By the time you've purchased or made a bench, wood, screws, fixings etc only to find that the lathe is simply not up to the demands you are placing upon it, you would have been better off buying a heavier duty lathe. Electronic speed control, down to the low hundreds, will also make a massive difference; if nothing else because you can raise the speed slowly, instead of pressing the start button and standing back to see what mayhem results.
Remember, just because a lathe has a 12" swing, it doesn't mean it can handle large out-of-balance 12" logs.
If funds are a problem, I suggest you concentrate on smaller stuff for the time being, and try to use only well balanced blanks until such time as you can afford a better lathe.
Above all else, remember to wear face protection, especially important if you are turning large gnarly lumps of wood of unknown properties - you can not duck quickly enough to avoid the unexpected missile. DAMHIKT!
Safe turning
Duncan