Q: Can someone tell me where to find OSX upgrade limits for older macs?

http://www.everymac.com

The following may help:

System requirements for OS X Lion and Mac OS X v10.6 - Apple Support

OS X Mountain Lion system requirements - Apple Support

OS X Mavericks system requirements - Apple Support

How to install OS X Yosemite on your Mac - Apple Support

       Macs that can be upgraded to OS X Mountain Lion or later

         1. iMac (Mid 2007 or newer) - Model Identifier 7,1 or later.

         2. MacBook (Late 2008 Aluminum, or Early 2009 or newer) - Model

             Identifier 5,1 or later.

         3. MacBook Pro (Mid/Late 2007 or newer) - Model Identifier 3,1 or later.

         4. MacBook Air (Late 2008 or newer) - Model Identifier 2,1 or later.

         5. Mac mini (Early 2009 or newer) - Model Identifier 3,1 or later.

         6. Mac Pro (Early 2008 or newer) - Model Identifier 3,1 or later.

         7. Xserve (Early 2009) - Model Identifier 3,1 or later.

To find the model identifier open System Profiler in the Utilities folder. It's displayed in the panel on the right.

Upgrading to Lion

If your computer does not meet the requirements to install Mavericks, it may still meet the requirements to install Lion.

You can purchase Lion at the Online Apple Store. The cost is $19.99 (as it was before) plus tax.  It's a download. You will get an email containing a redemption code that you then use at the Mac App Store to download Lion. Save a copy of that installer to your Downloads folder because the installer deletes itself at the end of the installation.

     Lion System Requirements

       1. Mac computer with an Intel Core 2 Duo, Core i3, Core i5, Core i7,

           or Xeon processor

       2. 2GB of memory

       3. OS X v10.6.6 or later (v10.6.8 recommended)

       4. 7GB of available space

       5. Some features require an Apple ID; terms apply.