Without having a version of Illustrator the same as or later than the version used to create the .ai file, there's not much you can do. I haven't come across any tool that will give you an accurate conversion of any but the simplest file.
Even with Illustrator, a direct conversion to an "equivalent PSD" (and I'm not entirely sure what you mean by that) is not a straightforward task. In essence, you would be building a PSD from scratch by importing the elements from Illustrator. You can't just copy and paste a text object from Ai to Ps, for example. Vector objects can be pasted in as Smart Objects (which will retain their appearance), or as shapes (which generally won't).
You may be able to use the imported .ai (Smart Object) as a template to build a matching PSD on top of it, but because the feature sets and possible effects are so different between the two programs you may find there are some things that are very difficult to reproduce.
From the question, I'm assuming that you need to end up with a fully editable Photoshop file. In that case, even if you have Illustrator available, there are almost certain to be one or several elements in the original that you would have to recreate in Photoshop, but using Photoshop's tools.