Just a quick answer before Christmas breaks loose:
The cost depends very much on how much you can do yourself. The biggest saving is from making your own sail. Sounds pretty daunting, but it isn't that bad. If you even can make the mast, that will save you quite a bit, but there are good mast makers in Scotland who can help you with that, at least one:
I am talking about JRA member Pol Bergius. I guess a wooden mast would need to be around 22cm diameter.
Cheers, for now,
Merry Christmas etc. etc.
Arne
PS: You would be better off with a rudder on the transom if you are to fit a sloop JR on a Vega.