The 8-core 2007 Mac Pro should be able to handle 1080p w/o problem. You can probably even work in H.264 natively for some of it (w/o having to transcode), depending on the bitrate and compression, and how complex your edits get. The most efficient way to do that would be Final Cut Pro X, which can automatically transcode to ProRes in the background seamlessly if you determine you need to do that. Looks like you've edited it out, but if you are indeed going to 10.11, you should be able to run FCPX 10.3.x, which is not the latest, but still very good and does most of what 10.4.x does. One thing you'll need to do tho is to upgrade to a supported video card for FCPX. A Radeon HD5770 would work well and be a good, inexpensive choice.
sounds good, depends on buget for upgrades
final cut X is super fast & a relay good option for editing (£300 in uk), you will need to replace the GPU
list of suported GPU's https://support.apple.com/en-gb/HT202239
You'll be fine with ProRes Proxy if you encounter performance hiccups (or just use original media and see how it goes). Also you can click the "View" button in the viewer and select Better Performance instead of Better Quality if you're adding many effects and titles and have frame drops.