Need Help - upgrades for my single speed and rebuilding an old raleigh grand prix

Hi Everyone, first-time poster. I currently have a Kilo TT that I’ve been using since 2014 as my daily driver. I recently inherited a 1980s Peugeot that has been converted to a single speed and before that, I grabbed my mother in laws old Raleigh Grand Prix from her basement with plans to convert that to a single speed(link is external) as well. I’m about to make some upgrades and want to swap some parts around, in particular wheels but I don’t think it’s going to be as easy as I hoped. I need some help.