HKB,
Read as much as you can, shoot as many different ones as you can, and join a local trap club or league/hang out at an ATA shoot.
Assuming that you'll be wanting one gun to eventually shoot all three disciplines in American trap (singles, handicap, doubles) even for fun...
New:
You can't go wrong with the numerous cx and 725 offerings from Browning, a Beretta 682/686/687 O/U or the like, especially when fitted with an adjustable comb.
If you're wanting singles/HC only, a single barrel trap gun like a BT99, or used options like a used Perazzi TM-1 or TMX are Also good choices. Remington's old 90T is another good gun in your range.
A used combo set can be had for around 3k if you look in the right places.
Last, but not least And in echoing The others - older or even newer Remington 870 trap models (TB, TC, All American Trap), the 1100 in its various trap grades thru the 1100 Competition Sporting, Beretta 303/390/391/ in trap configurations (don't forget a shell catcher if you're next to people and/or are competing), the Beretta A400 in step rib or multi-target variations, and the old Winchester Super X1 And Model 12.
Even in an age of high dollar SBTs and O/Us, Pumps and autos will serve you well for singles and handicap, and doubles if you've got the will to practice with a pump and a deflector on the auto. If it works for you, it works for you.
Like the others said, fit plays a big role in the shotgun sports. And if buying used/older, know what to look for.
Fun sites to drool:
Joel Etchen Guns
Pacific Sporting Arms
For what it's worth, my first 25 was with a stock Benelli Montefeltro, and my 1st 100straight was with a simple, stock 2008 manufactured BT-99

I also like flatter shooting guns up to about 60/40 since I also shoot sporting and skeet and I don't like high patterning guns.