Coleman are not bad tents. They are just not terribly light.
A few years back when I was car camping up and down the west coast, my cheapo tent from Big 5 sporting goods utterly failed in Point Reyes. I got soaked.
Picked up a Coleman '4 person tent' someplace north of Santa Rosa. Which is really a 2 person tent. It has held up really well.
But like I said, it's no ultralight. I hiked in to one campsite later in that trip and it made a huge difference in my pack weight!
As far as adding a tarp for a rain fly. Make sure the sides of it clear the corners of the tent. In other words, like a roof on a house it has to stick out over the sides a bit. The more the better.
Otherwise its basically funneling all the water it catches onto one area of your tent.
It's also a good idea to rig it separately if you can. Most tents aren't designed to be load bearing.
Sometimes I just rig a line between trees or the car and a tree. Feed teh grommets from one side on the line and stake the other grommets int the ground. It makes a big square with one end up high. Then I pitch the tent under that.
Don't forget a footprint or ground tarp under the tent. This really helps keep ground water from seeping in.
But that opens up the whole topic of drainage, flood plains and reading the terrain.