I'm not a Nikon specialist, but according to lots of articles, Nikon is really good with low noise( high Iso),
BUT doing a lot of night photography and long exposure, I can tell you (and you probably already know) that for this kind of photography, the lowest ISO, the better photo.
High ISO is good when you need to take relativly quick shutter speed but for light trails and star trails, either you use a low ISO or you blend several pictures( considering you're not doing it just in a bulb shot, i.e 49minutes)
I'm using a 450D and a 5D mark II and both have great quality about ISO performance.My gallery contains lots of examples of night photography or long shutter speed photography.
I started photography with the 450D and would recommend it again and again, as a evidence of it's great quality, look at it's price, which didn't get down for so long( at least, not in France and Germany, tell me if I 'm wrong about England).
the 500D(never used it) as even more capability so I believe it's a good choice.The Video capability is ( as photo mags say) under the possibility and quality of the 5dII.
I have to say I almost never use it, I didn't buy for video but really for photography.
About IR, I'm shooting IR photography with my 450D as well as with the 5DII. With a R72 Filter.
I would recommend you to send your "old" camera to it's brand service or to a camera professionnal to remove the " anti-Ir" filter on it, allowing you to shoot only, i said ONLY, IR picture with it.
the "rest" of photography via you new camera.
About price for the future accessories, Canon or Nikon, same battle....
It all depends what do you want as accessories...
Hope this helps, if any further questions,I'll try to answer