Am I correct in assuming that the SIG is not really a "true" 1911 and some accessories will not fit it?

Not really, it's as much a "true" 1911 as every other modern rendition that still has the same trigger mechanism. As for accessories...what accessories are you talking about? Sights will vary, but grips and grip screws etc should be the same as well as other parts.

What about holsters for the SIG?

How much does the rail on 1911 influence holster choices?

This is a combination issue, the rail and the profile of the Sig slide on that particular model (Sig recently released several models this year that have a genuine 1911 slide profile) can make holster fit difficult pending on exact holster. It's best to check with each maker and see what they say about the Sig 1911 line and their products. The rail will be an issue of itself, but most holster makers these days offer products to accommodate a railed 1911, I have holsters from Alessi and Milt Sparks for my railed 1911.

Can anyone attest to the SIGs Trigger?

Don't worry about it. People obsess over "1911 & trigger" way too much in my opinion. I've seen some people swear up and down that "Oh my word, this thing MUST have a 2lbs trigger pull!" only to be shown that it's really 4lbs, in short most people really won't know the difference. If you've never owned a 1911, you probably won't know the difference either. All that being said, trigger feel on a brand new gun can be deceptive as the parts will break in, a good cleaning before hand will remove any grit etc.

As for the quality of the trigger in general, Sig uses pretty good parts and the examples I've had my hands on have had triggers that were plenty good.

As for the Sig vs. Kimber....I'd go Sig as when it comes to firing pin safeties I prefer the Colt style used in the Sig as opposed to the Swartz model used in the Kimber. As I prefer neither of those types I'd actually suggest the new S&W E Series railed 1911.

One thing the Sig has going for it is the absence of the ever useless full length guide rod, I don't know why makers insist on putting that goofy thing into perfectly good pistols.

Going back to the Sig vs. Kimber, I'd probably opt for the Sig as there's less MIM parts in their guns, while some don't have a problem with MIM some do and it's something I'd rather not worry about. If I can avoid MIM parts, I will. Sig and Kimber both have some history in poor QC but overall Sig seems to do better in the customer service dept.

Early on the Sig GSR line had some serious issues that were traced back to bad batches of slides or frames, I don't remember which at the moment.

Before you buy I'd contact some holster makers and see what they say regarding the Sig and take a look at other options such as the Springfield Operator, Colt Rail gun, and the aforementioned S&W E Series.