the 50cc restricted mopeds are dangerous, you need to be able to maintain traffic speed
in town or city where top speeds are limited anything above 70 cc will probably be enough poke, not a sports machine but able to maintain presence in traffic without cars trying to squeeze past you because you're holding them up. 90cc is enough to get you upto 50mph eventually. For 40mph+, dual carriageways and open roads get a 125cc.
I'd recommend the CG125 or other commuter type unfaired sit up style 125.
Sports machines are asking for trouble and the fairing panels will cost a fortune when (not if) you drop it.
You dont just need a jacket & lid for protection. Make sure you've got good boots that protect your feet (heavy soles that wont crush), ankles and up the side of your legs. Good gloves and bike trousers that protect thighs, bum and knees (get CE knee pads, forget the sliders). Use the helmet properly, making sure it fits and is done up properly and don't muck about with dodgy visors, replace them when scratched (hell at night when all you can see is bright stars everywhere)