A varnish will help protect once painted i would leave at least a day for the paint to cure, a gloss varnish is best for putting decals on it helps the film around the decal blend in see at the top of the main pages about putting decals on, then you can put a coat of varnish over it to get the finish you want gloss, matt or satin.
at the moment i use humbrol clear found it to be good I've still not been able to find the floor polish that some people use (other people will be able to help point you in that direction), A few light coats of humdrol clear is better than one thick coat as its self levelling so it can pool in places if it goes on too thick, a problem I've had in very hot humid weather is that it can dry blotchy and white so i leave the varnish off till the temperatures have cooled down if it has happened usually another coat of varnish has sorted most of it out.