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CS5.5 should be able to open ID 2.0 files, but you'll have trouble finding a legitimate copy of CS5.5 anywhere.
The current version is CS6, with the new cloud-only subscription model version InDesing CC due in a matter of weeks. Your current version does not qualify for any sort of upgrade pricing (nor can it be used to activate a CS5.5 upgrade version, if you are thinking of tryoing to find one of those on the auction sites).
You can't buy CS5.5 anymore. The current version is CS6 with CC soon to follow.
Yes, newer versions of InDesign should be able to open all older versions.
Yes, InDesign can open any older version's files.
I would hurry up and buy the latest version CS6, while it is still available. The "Creative Cloud" version, launched in June must be RENTED on an annual basis, charged every month. If you stop renting, you haven't got an app to open your files with. Given that you haven't upgraded in several years, buying CS6 will represent much better value to you.
You'll find lots of very useful enhancements and features since ID 2.0.
What's the rush? CS6 will continue to be sold indefinitely. It will also
cost about $700 for a single app.
If the OP goes to CC for a single app he'll spend $240/year and get
upgrades along the way. It will take the better part of three years to
make that difference up and it's a lot easier to come up with $20 than
it is to come up with $700. Just something to think about.
If the OP has been using ID 2.0 for ten years, I'm guessing upgrades aren't a major concern. Creative Cloud use for ten years would be $2400.
While Adobe currently says that CS6 will be available "indefinitely", it's unrealistic to expect that it will be offered forever. Get it while stocks last!
Honestly, if InDesign 2.0 has worked this long, I'd imagine that
something along the lines of scribus would suffice. Ten year cost: $0.00.