I wouldn't get hung up on a Puch or Tomos unless you really like them. There are lots of great mopeds, plenty of parts available, almost all are fairly simple. (Moby, Peugeot, Vespa, minarelli, Sachs...)
There are some that are never going to be easy to upgrade.-For example an AMF roadmaster with a McCulloch engine. Even those are fun and will force you to learn some basic tinkering.