Here's my suggestions
If you plan to carry:
1. Carry Rig: This includes a quality belt, holster and mag pouch. I use both Milt Sparks and Kramer, but there are a lot of quality products on the market depending upon what you want to spend and what you want to carry (leather v. plastic).
2. Formal Training: If you have no formal training with handguns, take a class. There are lots of schools and lots of opinions about them. See if your local range offers a basic handgun course. it won't be a waste of money. After that, seek out some basic defensive handgun training.
3. Combat flashlight: I like the Surefire's, but there are several worthty brands. This item is essential for night time use and (as a member of this forum reminded me a while ago) in case the power goes out inside a building.
4. Night Sights: There are so many brands and some many opinions. I have the Sig Night Sites on my 239. I like them, but I think I can do better and are either going with the Heinie Straight Eights or the Novak bar/dot. I have read that it is faster at night to align the sights vertically than horizontally. I don't know if this is true or if it is really about training.
5. Lots of Practice: take everything you have learned and put it to use. Try the range at first and get comfortable with your firearm. They try some IDPA or IPSC or whatever contests may be offered in yor area.
Finally, I have the short trigger on my 239 and I love it. It is purely personal preference however, as I have small hands. As other's have said on this thread, extra mags are good also. I keep my new mags for carry use and my old ones for the range. As for standard capacity mags (for my 226, 228 and 229) I like to use 10 rounders at the range and in practice and save the standards caps for carry use only.
Best of luck and be safe.
Coley