Happy Monday.
I'll provide a critique to your list. I'll point out what is being picky and being legit:
1. Chassis: School or school-based transit M.Y. ~1982-2004, Rear Engine, 32-40' length, from an inland arid locale (or imminently inspect-able locally)
2. Engine: Cummins 8.3/ISC or International 7.6/DT466 (Maybe a DD6-71...maybe), sub 200k miles/7500 hours.
3. Transmission: >Allison 2/3k series OR most standard/manual transmission options i.e. Spicer/Eaton-Fuller (Stick would be a bonus treat for me personally). We plan to do plenty of highway/mountain driving so an AT545 or super-high ratio rear end is OOTQ.
4. Tires: No older than 2021 has been my basic cutoff. TL;DR tire age doesn't appear to translate to savings at auction. This is a surprisingly huge swing in actual cost to buy. I have seen otherwise great buses with "BAD TIRES" close for upwards of $7k, which means that was actually a ~$11k minimum purchase after replacing $4000 in tires. Maybe that bus was worth that amount in a vacuum...but I've also seen buses with way newer tires go for the same amount or less...it seems tire health is disproportionately important to the overall value of the bus.
Answer to 1, Chassis:
Correct.
Answer to 2 Engine:
Correct, but you can include T444E. It's a solid engine as well.
Answer to 3 Transmission:
Also correct.
Answer to 4 Tires:
2021 is a picky cut off date. I run 14 year old tires on the rear. 10 is cut off date for steer tires if the tread is still good simply because dry rot will start to set in and crack. A lot depends on front steer tires. If a rear blows, it's scary but usually okay, you still have 3 other tires.
'Nice to have' items:
1. Undercarriage storage - Pass-thru setups are really nice but we can build our own if necessary. We would probably end up adding additional bays even on buses with full storage just to make maximum use of the available space. This is one major benefit of the RE platform without a driveshaft assembly cutting a big trench out of your useable underbelly space.
2. High roof - As much as I had told myself I wouldn't do a roof raise on the first bus, I have softened that stance over the last few weeks as I learn about the process and build up my confidence with tasks like welding & riveting. So, a lower ceiling is no longer such a dealbreaker item if everything else looks good. I can substitute elbow-grease for funding here which is how I like to live.
3. OTR AC & Factory diesel heater (Webasto) - While it would be nice to have this piece of the puzzle in place already, adding an aftermarket diesel heater is not particularly difficult. I've read that they tend to be more efficient as well. AC has any number of solutions although it seems keeping cool while on the road is a particular hassle.
4. Continuous rain gutter - I really prefer the long continuous rain rail as opposed to the individual window bump-outs. It seems the transit-style chassis more often got the continuous rail, and it looks/functions more like a gutter on a house. Eventually I'd like to have rain chains and a collection system. We have toyed with the idea of having some kind of greenhouse space in/on the bus. If our filtration system is robust enough, we could even plumb it into our main water system.
5. Transit-style windows - We intend to retain as much glass as possible for maximum natural lighting. Many school chassis were also built in a transit variant or for some other adult purpose and these seem to have upgraded windows more like what you might see on an RV or coach bus. I'm not sure if they are double-pane or have any actual benefit over the school-style windows, but they have to leak less, right? Plus they are simply bigger. With a roof raise it may be totally irrelevant.
Answer to 1 Undercarriage Storage:
Not necessarily so. Dog nose have space as well it's just in the rear instead of the middle, however RE models do often already have the storage compartment pre-built where as dognose you have to build it yourself, and dognose has the driveshaft in the way so you have to build your compartment around that, so RE is still slightly nicer, but I wanted to correct that space wise it's about equal but does require more work to utilize.
Answer to 2: High Roof:
That's the spirit. I'm doing a roof raise as we speak. My first bus as well. It's good to push yourself and you learn life skills which can help you. I built a 20ft trailer from a rusted frame and welded it back together. I learned welding from a trailer first, now I get to apply that skill to the Bus.
Answer to 3: OTR AC & Factory Diesel Heater:
AC is nice to have, and it's difficult to add. Easier if you have a Dognose as you have more space to mount the mini-split condenser. where as you may have fitment issues on a RE because your engine is where you'd want to mount the mini-split. Diesel Heaters work great as after market and are quiet easy to install. It's a non-issue if you have to get one of these for either type of Bus. They heat well as aftermarket. Factory AC is best but you can add mini-splits.
Answer to 4: Continuous Rain Gutter
This is an interesting concern, one that most do not even consider, but I'm glad you do consider it. My 30 year old dognose bus has continuous rain gutter and it's protected the windows for 30 years. No leaks when I opened up the skin. Zero rust under them. Many 2000+ year buses do not have these, and they will leak if not continuous. I can attest this is a good ask, but you are likely gonna narrow down and limit you to mid-90's buses for this option. You could potentially get sheet metal and make you one of these on a bus that does not have them to open up your options.
Answer to 5: Transit style windows
This is preference. School buses let in a lot of natural lighting but also a lot of temperature as they are not double paned. This also requires you to likely skin the sides where the windows go and if you are going that far you might as well roof raise. Window skinning is half of the roof raise process. I decided even with a roof raise to keep my windows since I have the Continuous Rain Gutter and I know they won't leak, but I will pay for it in temps lost. I counter temps lost by having 4 inch insulation floor, and 3 inch wall and ceiling. It will balance out from the window temp loss, but I can improve the windows with a slight tinting. should cut about 10 -20 degrees out. I'm an odd man out as my choice to keep windows isn't as popular because traditionally they are more likely to leak.