1. First of all you can't upgrade. You'll need to buy a full version or move to Creative Cloud. If you only open those documents they SHOULD be fine, but the second you make an edit the entire story is very likely to reflow due to differences in the text engine.
2. Not likely at all.
My husband is wanting to purchase InDesign to transfer his PageMaker documents, but is not sure which version to get. He has a Mac Book Pro with Snow Leopard, OS X 10.6.8. Processor 2.4 GHz, Intel Core.
We are looking at InDesign CS3. Do you know if this will work?
(I tried posting this question in the question bar, but it would only do a search for me. I hope it is okay to post my question here).
The only version available now from authoized resellers is CS6. You might find a copy of CS3 on an auction site, but be very careful. Educatioanl versions cannot be transferrred, and commercial version transfers require the original licensee to deliver a transfer form and all copies of all versions iin an upgrade chain, so very few copies of older versions are legitimate and registerable. You might want to look at Creative Cloud. I's a subscription, but quite a good value if you will use more than one application from the suite.
It sounds like you are still on a Power PC, maybe a Mac G5?
As Bob pointed out, since InDesign CS came out in 2003 - 9 years ago - you are out of the upgrade cycle.
However, even if you decide to pay full price for InDesign CS6 you will need an Intel Mac and I believe that the 1st generation of Intel Mac - the Intel Core Duo, won't work.
CS6 needs at least a Core 2 Duo or better, so picking up a used Intel Mac wouldn't work if it was only a Core Duo.
Creative Cloud subscription service only runs on Intel Macs so if you are still on a Power PC it won't be an option.
If you are on a Mac G5 Power PC you can only upgade to Mac 10.5 Leopard, If you are still on a G4 processor, it requires a minimum clock rate of 867 MHz, and at least 512 MB of RAM.
I hate to say it, but by resisting upgrading you are facing two unbridgeable chasms: the Power PC vs Intel Mac & the lack of any affordable upgrade to CS6 vs Paying full price for CS6
Yes, I do have it currently installed on a G5 PowerPC, but have an MacBook Core Duo 2 that I could install on. The full price on InDesign CS6 is not the major cost. It's the 100's of hours of editing needed to clean up the mess created by the changed text engine in the new version. This is why we have avoided upgrading and will continue to as long as we have a computer that can run InDesign CS.
Using the single-line composer rather than the paragraph composer might mitigate that for exisiting files. I suspect, too, that the amount of cleaning up will be far less than you fear. Pagemaker was far more differnt from ID than CS6 is from CS in terms of the text engines and you've already fixed the majority of the pain points (different methods for tracking, for example). Your more likely just to see occasional differences in lie endings within a paragraph and occasional line count differences, which are certainly serious, but in general pretty manageable and easy to spot by scrolling through the files looking for oddities at the bottoms of pages or overset text. I think you'll find more cases of reflow with justified text, too, than with ragged, but I wouldn't swear to it.
On the whole, once you get used to the newer version, I think you'll find enough productivity increase to offset any rework time.