You know - if Merckx had ridden the bike in a race, you could worry about preserving something of historical value. Since there's no indication anything of the sort is the case - do whatever you want with the bicycle. If you care about originality, all this advice you've been getting is very close to the mark. If it truly is just transportation to you, paint it up and build it up however you want - taking into account the advice given that will help you from making a mistake that could threaten the integrity of the frame. You've got a nice 531 frame, and that's an excellent starting point. Have fun working on it and enjoy riding the bike.