What kind of groupset are you talking about? The thing with the cooper is that it has no braze-ons and it likely spaced at 120mm in the back. Shoving a 130mm wheel in there could be done, but you should not do it, IMHO. You're likely to need tektro long reach brakes also which will add cost because whatever comes with your group will not work (if you are talking modern)... it's going to cost you more than $500 to put together a modernish build on the cooper and I think most people are going to advocate you go vintage. Do you want to ride vintage components? I think if this bike fits you, you should build it up as a single speed or fixed gear and also buy the felt.