Try asking here: http://www.rugerforum.net/
I was able to pick-up a Ruger P90 for a real good price. I upgraded the stock grip with hogue grips. What upgrades to the trigger/hammer springs can or should be made? I think I will also change the sights out to a brighter set-up.
What is the pistol doing that is wrong, and that would require changing the trigger/hammer springs?
If it is used and has a high round count, then you might replace the recoil spring. Wolff springs have a great reputation, and they have a web site that makes it easy to order just about any spring you might need.
The only rounds through the gun has been what I shot in it the other day after getting it. I was just figuring with the price I got it for I could put any upgrades on it and still be okay with the wife. LOL
I'd suggest a good holster, a couple of spare mags, and a case of ammo. Perhaps a sight upgrade, but unless you want/need night-sights, the stock sights are better-than-average for non-critical uses.
I had a P-94 in .40 S&W and it was built like a tank, usual Ruger ruggedness. The sights were excellent out of the box and if you wanted to upgrade to night sights, that would be a personal choice. Get as many spare mags as you can for ease of range time. One thing I do prior to going to the range is load all my Beretta mags and bring the gun in empty as required. Saves significant time preparing to shoot. I have the weapon loaded for S.O. qualification when I go, but that is not to a public range. As suggested a good quality holster and mag pouch would be a good investment for the weapon, are you going to carry it CCW? Shoot often and become proficient with the P90 as you are able. Take some extra money and invest in quality instruction such as Front Sight or Gun Sight classes, great investment in your future as a shooter.
The P90 comes ready to use out of the Box. As for Customizing, its up to the Individual's likes. This one has had an Action Job by Teddy Jacobson, Engraved by Orval Mahan, Stainless Steel captured Recoil Spring and Buffer from Fire Dragon, and Stag Grips by Pat Grashorn.