I can't speak in detail too much about installing the Soundtoys Tremolator plug-in as the system that I have used it on already had it installed on it, so I wasn't involved in the installation process. However, I can say that while I have used this plug-in that I haven't had any problems related to installation or otherwise. The interface of the software isn't hard to follow as it is laid out in a user friendly fashion. The plug-in has parameters for depth, groove, accent, shape, rhythm, tap tempo, rate, and for input and output levels. After playing around with the plug-in for a few minutes I found that I was able to understand everything about the plug-in pretty easily, although I'm always finding out new combinations of how to set these parameters in order to get different sounds. I don't think that a manual is necessary but I can't say for sure because I've never seen the manual and can't say if it would be helpful or not.
I have used the Soundtoys Tremolator plug-in on an Apple G5 computer that runs with a Pro Tools HD system and I can't say that I've ever had a problem running this plug-in. This isn't a plug-in that I have ever had the need to run more than two of at a time though, so I can't say how well it would run beyond this but I would imagine that most people wouldn't be running too many of them in a single session anyway. How well you can run this plug-in largely depends on the system that you are using, but if you are able to run the plug-ins you have now well enough, I don't think that this one will give you any trouble.
Overall, Soundtoys Tremolator is a very solid plug-in. It has a great overall sound and provides a lot parameters so you can really get all sorts of different tremolo type tones. The price is reasonable as well, so if you are looking for a nice tremolo plug-in to add to your suite, this is absolutely worthy of a trial. There aren't too many other tremolo plug-ins out there that I would rather use over Soundtoys Tremolator.