2. Maybe…maybe not. CS4 is unsupported.
3. That should not be an issue.
You should note that CS4 has not been sold through any legitimate channel in quite a few years. The likelihood of buying a genuine copy of CS4 is very low.
I hadn't considered that the versions in ebay might not be legit. To your knowledge, is there another earlier version that is more readily available?
You can still purchase CS6 as a perpetual license, but it doesn't make a lot of sense to do so -- it's not inexpensive, it won't see any new feature updates, and it won't be supported on newer operating systems for much longer.
As Peter says, you’d likely be throwing good money after bad. A subscription to InDesign though Creative Cloud is $20/month while buying CS6 (which is included in the subscription) would be somewhere around $700.