Vessel sizes are fine.
We've gotten samples back from the mash coming straight out of the cook. We use a grain to water ratio of roughly 3:1 ratio. This size operation I'd assume you'd want to run either 3" or 4" pipe, I certainly wouldn't go smaller than that.
I imagine a big plant would be crashing the cook temps from ~140 to fermenting temperature with a once through tube and shell exchanger.
Through a 3" line, the apparent viscosity of our mash is 7.2cP @ 140F, and 15cP @ 80F
Through a 4" line, the apparent viscosity of our mash is 13cP @ 140F, and 35cP @ 80F
I don't think 4 hour cook times are achievable with that volume without DIS. For reference, our runs are 2500 gallons, (1/5 your size) and with DIS we can run it up to 200F in about 2.5-3 hours. You can cut down on that time if you are able to recycle some of your heat from other processes.
I would run a central top mixer with props and run CIP spray on either side. CIP spray balls (the size I'm familiar with anyway), won't be getting that thing clean if you're only using one and running about 14' diameter. It also eliminates a number of problems that running from the bottom induces.
You can run without a grist hydrator without a problem as long as you're careful with you pH and temps, but I imagine it's a worthwhile time saving investment.