I carry a Ruger P-90 and although it is not the identical gun you asked about, it is really close since they both have the same design.
The newer models do have a picante rail system on them so you can put a flashlight on it. The picante system is fairly common so you shouldn't have a problem finding a light. Older models (probably older than about 3 years) won't have this feature.
Another change to newer models is that the grips are now a molded part of the frame and you can not change them. You could add something like a slip on Hogue grip however.
Since I bought mine used, I can't make any comment about break in period but I wouldn't think it would be more than a hundred rounds or so. Milage may vary.
Holsters are a personal preference. I have tried a pager pal and didn't like it. Shoulder rig is ok, but mostly I use a Galco knock off. I am starting to look for a Fobus holster though...hard to find since the P-90 is no longer in production.