Dear Chris,
I have just read your thread on the ML7 and most of what has been said there applies to your S7 however personally I would not bother converting. But if you do I think the DRO is the way to go and there are plenty people who have done this and very successfully.
I've been cutting metal for over 50 years now and everything I own is imperial and the "on board" computer was programmed in imperial, it even has to do a quick computation of anything metric to give a meaningful value. I realise the younger generation will be all metric and wonder what inches are but for me I will stick to that which I understand. If I get a metric drawing a few moments on the calculator and I have all the dimensions in imperial–Oh joy 
Regarding screw cutting, I think you said you have a QC gearbox on the S7 ? If so to cut metric you will need to buy the kit from Myford which consists of a new quadrant and some additional change wheels —will have to look at mine to see which ones. This will allow you to cut a range of metric pitches just by changing the levers on your QC box but you will have to leave the clasp nut engaged for the duration of the operation and reverse the lathe back to the start point after each cut.
There are several cleaver devices that have been designed to overcome this if you want to cut a lot of metric it will be worth considering building one. look at "screw cutting made easy" on this site.
I also have an Emco Maximat and the conversion for metric threads is much easier on this machine so if I REALLY have to cut metric this where I do it now.
Wishing you success in your endeavours whatever you decide to do.
Regards John
PS where are you located? I am in Lancashire