The joker in that deck will be if the 7-speed freewheel fits between the dropouts. I've done 6-to-7 conversions that just required the shifters and freewheel to be changed. but I've done others that required the wheel to be re-spaced and re-dished. And a few that were neither. You won't know untill you get the parts together and try.
If you do go the 7-speed route, that 11-28 Shimano freewheel is a great item; but IIWM, I'd do the 6-speed and shifters and ride it like that for the season. Then if you decide you love the bike you can do the 7-speed/brifter conversion next year when you know a little more.
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You know it's going to be a good day when the stem and seatpost come right out.
(looking for a picture and not seeing it? Thank the Photobucket fiasco.PM me and I'll link it up.)