I need HELP

I just bought a 95 grand marquise last week n the
guy told me the the radiator would need to be
replaced soon but it wasn't bad. Its a few minor
things that's wrong with it. A friend of mine looked
under the hood yesterday & did a little nurturing to
her. When we popped the hood my pipeline
connected to the radiator was causing pressure n
making her overheat but not too bad. So when he
opened the line it was corroded with rust n no
telling if they ever put coolant in it. Just water n it
hasn't had that in a while until I got her. We also
realized the thermostat was taken out.So I went to
the nearest AutoZone n they told me to get a
thermostat, 3 day flush, n coolant. So he put the 3
day flush in n told me to drive it for all 3 days.
Today was thunderstorms all day so i didn't go
anywhere, but when the rain died down I went
outside to crank her up n I started seeing alot of
smoke non-stop & in less than 5-6 minutes she
was overheated full throttle n she has never done
that before. She was overheating but not that fast
n not that hot. Plz help, what can I do???