i did mine with off the shelf car touch up spray.
some things to consider, make sure you can get more of the same colour and brand of paint,
a thin tacking layer is needed when you first start painting, after that you need it to be going on almost liquid so that the chemicals in the paint can allow it to form with the layer below instead of forming a seperate layer on top.
two starting points:
with rust and muck I'd go for sand blasting then primer, sanding down nice and smooth between coats.
a nice, none rust coated and just slightly tatty paint, heat any decals with a hairdryer or heat gun and peel them off,
then sand with wet wet and dry paper, gradually getting finer grade so that its almost smooth,
a spot of lighter fluid on a bit of kitchen towel to clean anything off it and wipe with dry cloth.
start spraying with desired colour, thin coat then layering it on so that it looks wet, this avoids a powdery finish.
tis how I did my wife's bike