Lovely frame! Nice price, too. It's a 1981 Raleigh Carlton Super Course.

First, I'd find out what the original groupset would have been and go for that if I liked it. If I didn't, I'd fit it out with Campagnolo of the right period as a thank-you for staying NOS for so long. FWIW, I think it had some of the same components as my 1980'ish Dawes Atlantis, which has a Japanese SR crankset, SunTour PowerShifters (friction with ratchet). My Dawes had a SunTour Vx GT derailleur but I think your bike would have had a SunTour Cyclone - both are good derailleurs. Both the Dawes and your Raleigh had Weinmann brakes.

However, it's your bike and you should do whatever you want. If you make it a single speed I would beg you not to grind off the droupout so that you, or whoever comes after you, can return it to its original purpose as a geared bike. I saw a beautiful candy-blue 3Rensho the other day that had been made into a fixed wheel but the owner had left everything - droupout, shifter braze-ons and cable guides. It was stunningly beautiful like that...