I've been considering the 123 too. They get good reviews but mainly from folk who have replaced old sloppy distibutors. The distrbutor I have in the 356 is grade A, tip top condition having being restored by Ed Falls at Vintage Werks and more recently looked at by Prill which wasn't cheap.
I was thinking there's no point in fitting a new fangled 123, my distributor being touched by the Great Himself (and Ed Falls) but then it occurred to me this would be an ideal test review.... my perfect dizzy against the 123 distributor. I'm convinced it's a good plan but like you say, it's not a cheap plan.
I digress. Your starting problem could be a timing thing. Have a go at setting the static timing accurately with a multimeter or test lamp and see how you get on.