I can't think of something much worse than an H&R Topper. It will kick the snot out of you and there's no worse adjustable pad than the Morgan.
Both points are true, BUT.....
I can't think of a cheaper gun to play with than the Topper.
The same holds true for the Morgan pad.
You could conceivably put together a capable trap gun for under $100 using jgalar's tips.
I would take a look for a used 1100 trap. Bang for the buck, it has it all.
Pros: Good construction, known performance, availability of parts, easy on the shoulder AND the wallet.
Cons: gas autos have to be cleaned more than O/U, singles, or pumps. Keep it clean and keep an extra o-ring in your bag, and you will only notice the "pro's" of the 1100. I love mine.
The 870 is a top choice for a pump. I love my 870 too, but my 1100 works better for me.
For a single, the BT-99 is a wise investment. The Spartan is a great bargain and will stand up well, but it's no Browning. That said, as a matter of economics, I bought a Spartan O/U, and was pleasantly suprised.
But like Bridger said, try as many guns as you can before you buy. Whatever you get, it has to fit YOU.