Upgrading to new rImac from much older OS; will Time Machine Restore be ok?

You can use your Time Machine backups to get your data onto your new riMac, but instead of restoring, you should either let your new Mac ask to migrate your apps and files using setup assistant (or Migration Assistant, in Utilities, if you end up skipping that step during the setup process). This way you're just bringing your applications and files over, not restoring an old OS on top of the current one on the new riMac.

I did a TM restore from a late 2012 with no issues. You should update all your important apps regardless to the latest versions available. Many fixes in general and Yos compatible.

Scaredpoet is right; you'll be migrating, not restoring. A new iMac can't even run on the old system, so I doubt it will even let you restore.

Migration works great. But that's a much older machine. You need to check to see whether your old applications will work with Yosemite. My CS 6 Photoshop does, but it all depends on how old that stuff is. And same is true for some hardware, like printers and scanners. You'll probably need to upgrade a lot of stuff besides just the system software.

In any case, most of your data should be fine. Photos in particular.