find whatever is cheap with a title, learn basic stuff first before $$$ out on expensive aftermarket.
ill go and say that a 505 Sachs-based moped is a good candidate, pretty simple, easy to get running (ish), and most importantly CHEAP! talking like 40-150$ for a good looking non-runner.
that or a honda hobbit or a Yamaha QT50, still relatively cheap and a decent aftermarket, also will run forever if taken care of.
Puchs and Tomos are kings, but command a scalping price for a beginner (like 500-600+$ for a beat to shit one) id say if you got the bling, or find a good deal, grab one of those. (edited)