go to Sig Forums dot com. Read a lot.
A couple of quick points:
The design is around 40 years old. The P6 [or 'original' p225, in civilian production] has a separate breech block, pinned into place. The breech block pins are scheduled for replacement something like every 3000 to 5000 rounds, according to the Sig Armorer's manual. There are two of them [inner and outer]. While in there, change the firing pin spring- why not?
The P6 has a generally heavier trigger pull than the P225. Generally associated with spring weight.
IF you like it enough, you can have a Short Trigger and a Short Reset Trigger [or is it Short Trigger Reset] installed. When I sent it off for the package to Sig, it dropped the DA pull noticeably, made the SA pull smooth/crisp, shortened the trigger reset, and the thinner [front to rear- aka: short] trigger was easier for me to control in DA mode, as my fingers are slightly shorter than average.
Mags are not cheap.
There is a guy at sig forums dot com who makes nicer mag baseplates for it, which look better and feel better in the hand. However, the factory one is better for 'duty use', as the extra length of the tab on the base plate allows rapid forceful removal of the mag.
There are aftermarket grips available: Hogue for G10 is pretty good. Marshcal makes a beautiful wooden grip- but I need thin grips.
It is possible to use a more recent hammer spring strut and hammer spring, to improve trigger feel, but I can't remember what the parts are- go to sig forum dot com.
The frame is aluminum and the slide is steel. Keep them lubed well.
No one is making aftermarket barrels for them any longer- BarSto quit about 3 years ago.
Parts availability is shrinking.
That is about it!