While the conversion kits are not cheap and sometimes hard to come by, they are worth the expense. Their most significant advantage is that they are not considered to be a firearm, so you can buy one and immediately start to shoot it without having to register it with the CFO.
SIG P226 frames are all mechanically identical (save for special editions such as LDC, DAK or SAO versions). Whether you have a 9mm, 357 SIG, 40 S&W, or 22LR, you can mix and match any standard P226 frame to any slide/barrel assembly with 100% reliability and function, and only you will know the difference.
Conversion kits come with only one magazine.