I had a 2003 Chevrolet Silverado LS extended cab short bed with the 5.3 liter V8 and 3.42 rear.
It also had no towing package. (I didn't even check when I bought it because I'd never heard of a truck without one before.)
We bought a larger popup (PUP) that weighed around 2,500 lbs. and I had a trailering package installed at the local Chevy dealer.
~ $100 for the hitch receiver and electrical wiring (make sure they put the 12v through the ignition so that you won't drain your battery dead when the truck is turned off).
~ $100 for an electric brake controller (I'd recommend this for ANY size trailer that has brakes). You can get a proportional controller (Prodigy or P3) by Tekonsha here:
Prodigy for $112.00
P3 for $134.00
You should be able to buy the specific wiring kit that matches your year, make, and model of truck for around $10 which will make installation a breeze. These are FAR superior to the timing-based controllers and don't cost much more.
I towed regularly with a 2" ball with no problems. I rarely used Tow/Haul mode or had to shift into 3rd. There was plenty of power and no strain on the engine - < 2,000 RPMs at level interstate speeds.
For good measure I added a transmission cooler for about $60 and a heavy-duty fan clutch for about $60.
This is where I should have stopped.
Instead, we bought a heavier trailer - closer to 5,000 lbs. and a larger profile which added more wind resistance. It didn't tow nearly as comfortable, so I added a cold air fuel injection kit by K&N for $225. Terrible mistake. I got a little more power, but it was negligible and certainly not worth the jet engine sounds I produced at wide-open throttle on hills or entrance ramps with the full load behind me.
I had upgraded the tire size one level from stock before I started towing without realizing that this has a negative impact on your rear end gear ratio for towing performance. Essentially, you turn a 4.10 into a 3.73, a 3.73 into a 3.42, and a 3.42 into an incapable towing machine. Right before I was about to change out the rear end with a 4.10 Positraction, I decided to quit trying to BUILD a tow dog out of a grocery getter and go buy a truck that was setup right from factory.
If you're going to stay with the PUP, I think you'll do fine to invest $200 to get the trailering package and brake controller. If you go on long trips, in a lot of traffic, or into hills, then eventually add a transmission cooler and a fan clutch. These are things that will increase the value of the truck a little as well if the buyer is looking to tow.
If you upgrade your trailer at all. . . consider a new(er) truck.
OK - YMMV. I used to get 20 mpg.

Now I don't.
*To clarify on edit*
I upgraded to a 2005 2500 HD Silverado LS crew cab standard bed with the 6.0 liter gas engine and 4.10 rear end. The fuel economy is terrible, but I work from home, so it doesn't hurt too much. A properly equipped 1/2 ton would have been more than adequate but I needed the full crew cab and 6' bed, so I took advantage of a dealership not being able to move a gas guzzler. I got a great education about trucks through my pocketbook. To be fair, I'm one of the conservative ones who likes to stay around 80% or less of the vehicle's ratings. And yes, I went to a truck stop and weighed the whole setup. I'm also that guy.
