I'm a vespa px excel owner and hav a great passion for vintage
vespas too. 
to my opinion, u need only abt $5 for full tank and regular
servicing to keep ur vespa in a good shape. of cos, the important
thing is to check ur brakes and tires too. its a very convenient
machine, and unique too. u can ride in style with own colours or
designs. vintage and chic.
on the expressway, always be wary of the lorries especially cos
sometimes they can't see u. and if it rains, do ride safely. it
might get slippery as the tires are smaller than other bikes. abt
speed, I've rode up to 100km/h. keep a safe distance and be in
control of the vespa.
if u wish to purchase a vintage bike, its advisable to bring
someone who's experienced or owns a vespa. sometimes cos its
vintage, there's a need to change the parts or battery. the fully
restored ones are not bad I guess.
hope this helps u in deciding which bike to get. fyi, this is just
my opinion and u should think carefully and be responsible for ur
own actions. gd luck for ur TP in future.
Love,
Dia
