Ask 10 trapshooters that question, expect at least 10 opinions.
And why not a pump? My 870 TB is a claycrunching death ray when I do my part.
I'm not enthused about a pure D trap singles gun for starters.While they're reliable and effective, they're a bit specialized. And the good ones are much more than $1K.
Lots of folks get into trap with a field gun, and use it as is or slightly modified.
You may want to try out some semi autos like the 1100 Remington, the Winchester X2s, the various Berettas, and so on. The gas actions cut the kick a bit.
I suggest getting one with choke tubes, for adjusting your pattern for optimum density and spread at the distance you're breaking birds at. Also, a tubed gun can be used for the other clay games, and while I like trap a lot, I've not found a clay game I didn't like.
Trapshooters are a pretty decent lot, tho some grumpy old men are a bit touchy. Ask to handle a few guns and ask their opinions. Since some trapshooters change guns and socks at about the same rate, some good bargains can oft be found through little notes on the bulletin board at a range.
HTH,,,,