Yeah, you should be able to use the same key. There's no activation or anything like that in 8.5. If you have the program open on two different computers on your network it will complain but that's the extent of it.
Where to down load the program from I couldn't tell you. You can always get the current version but I've found that old versions disappear as soon as the new versions come out.
You can buy FMP8/8.5 directly from FMInc through August, so I've been told by them. There may be a "downgrade" fee, though.
The best source of info for what's allowed or not by FMI licensing would be FMI itself...
Thanks for your post.
Support for Filemaker 8 and 8.5 will end on September 23rd. Until that time replacement discs and downgrades are still available. While prices are listed on our store page I will duplicate them here as well. Media replacements cost $20 US. Downgrades are a bit different. To purchase the older version you would need to first purchase the current version. Then contact our customer serveice department at 1-800-325-2747. There has to be a valid reason for the downgrade, it not an automatic option. Customer service will be able to confirm if a downgrade is available based on why you need it. Both of these options will become unavailable once the end of life date is reached. Note that neither 8 nor 8.5 is tested or certified on the newer OS version for both Windows or Mac, so if you intend to upgrade to a later OS version you will also want to upgrade your FileMaker version.
As for the license key that will depend greatly on how you intend to use the software. If you intend to have multiple systems accessing the same database on the same network then each machine needs its own key. If one is meant to be a backup installation for the same user then this is allowed under the license agreement and the same key can be used for 2 systems. More systems as backup would require additional licenses to avoid a breach of the EULA.
Hope this helps.