I built the 50x50 KRMx02 by Kronos Robotics. There is a 50x102 "version" you can build. There is a kit of electronics and a few mechanical parts from CNC Router Parts that is outlined when you buy the book. Just has a different design using more 80/20 and is more stout. Not going to go into great depth as you can look them up but there is great support from the author of the book via the forum which you get access to once you purchase the book. Either way you will be fine. The components are easily replaced if they wear out/break and you can afford to keep spares on-hand if something breaks.

The router vs. spindle debate will ignite a lot of just that, debate. Lots of low production CNC routers use handheld routers. Many of which are the 2.25HP rated models. As mentioned, bearings are CHEAP ($5 for my Bosch routers) and you can change them out yourself. Just keep a spare router and you have a very redundant setup. You are talking about some serious runs though so I am sure there is benefit in you going with a spindle. Just remember that if something goes wrong with any spindle it is usually not a DIY job to repair and unless you can afford to be down for a few days and/or have a backup spindle on-hand then there are some personal choices you have to make in regards to what you want to do. Some day I may upgrade to a spindle. For now, with the Super PID and a router I have been really happy. Most of the time I don't need to run the router at top speed where people complain about the noise of a router. Running it at lower speeds, appropriate for the material and plunge/feed rate, extends the life of the router and tooling.

The Nema 34 vs 23 debate is another one that I don't understand. Mostly because of my limited technical understanding of the difference. You get better resolution out of the Nema 23 kit and it seems to me that the 34 is only good if you are doing some heavy cutting for long periods of time. Not sure if your question about using it 8x5 is really how you are going to use it or if you just wanted to get someone that has used the design enough to stress test it. If you were using a router by hand before then your throughput is going to go through the roof with a CNC router so you must either need the extra capacity because of growth or you plan on running it for those hours in batches and don't need it to perform at that level constantly.

Whatever works in your budget. You can always upgrade to a spindle if you decide a router is not right for your work or upgrade the electronics if you are pushing the 23 too much. You seem to be just getting started with a CNC router so you are going to have A LOT of other decisions to make along the way. The frame/design is important but all of the popular designs are great. Just pick one and get started. I thought about it way too long and no good came from over thinking it on this one.