Whats the best laptop to buy so I can use my Final Cut Express HD 3.

Hmmm, looks like Yosemite might br the last OS it worked in?

https://roaringapps.com/app/final-cut-express

Compatibility

Architecture: PPC x86 (Intel:Mac)

• Macintosh computer with 500MHz or faster PowerPC G4 or G5 (550MHz for PowerBook G4) or Intel Core Duo.
• 512MB RAM.
• Display with 1024-by-768 (or higher) resolution.
• An AGP or PCI Express graphics card compatible with Quartz Extreme or an Intel GMA integrated graphics processor in MacBook or Mac mini.
• Mac OS X 10.4.6 or later.
• QuickTime 7.0.4 or later.
• HDV features require 1GHz or faster processor and 1GB of RAM (2GB recommended).
• 500MB disk space required for applications; additional 16GB of additional storage space for Soundtrack and LiveType content (can be installed on separate disks).

Final Cut Express HD 3.5, LiveType 2, and Soundtrack 1.5 are all Universal applications, optimized for Macs with Intel Core Duo processors.

Great news, I never did for sure what the top OS that would run it was, but there are some fairly fast Macs that can run Yosemite...

These are the models that are compatible with OS X Yosemite (with exceptions):

• iMac (Mid 2007 or later)
• MacBook (Aluminum Late 2008 and Early 2009 or later)
• MacBook Pro (13-inch, Mid 2009 or later; 15-inch, Mid/Late 2007 or later; 17-inch, Late 2007 or later)
• MacBook Air (Late 2008 or later)
• Mac Mini (Early 2009 or later)
• Mac Pro (Early 2008 or later) (Can run on a mid-2006 version if one upgrades to a supported graphics chip and utilizes a custom bootloader)
• Xserve (Early 2009)

These are the models that support new features such as Handoff, Instant Hotspot as well as AirDrop between Mac computers and iOS devices:[6]

• MacBook Air (Mid 2012 or later)
• MacBook Pro (Mid 2012 or later)
• iMac (Late 2012 or later)
• Mac Mini (Late 2012 or later)
• Mac Pro (Late 2013)

https://en.wikipedia.org/wiki/OS_X_Yosemite

Take care my friend! :)