Swann; excellent choice.
Check out Brownell's (www.brownells.com) for replacement sights. The rear sight - unless changed drastically since mine was made - just drift out and in of the slot in which it sits. Drifts to RIGHT side of pistol, by the way. Enters from right to left. Brownells has enough variations to keep you talking to yourself for several days.
I think they also have after market magazines and yes, someone makes an eight round magazine.
Shoulder holster? Galco makes an excellent rig. Be warned, most ranges will not allow use of any holsters - and especially shoulder holsters - while shooting. Many 'instructors' don't like shoulder holsters; not because they're not useful, but because the draw is seen to 'sweep' everyone on one's left during the draw.
Ammo? There are a couple of places on the web selling handgun ammunition. Some are very inexpensive indeed. Do a search for 'ammunition sales' and you will many from which to choose.
Oh. Take money.