Not sure why you are talking about 240 stuff here. The OD conversion is straightforward except for the tunnel mods and you're more than capable of that. Using a normal 120/1800 OD box it will more or less bolt in.
Comments on your list
1.True but can be cut and shut. see
https://www.youtube.com/watch?v=VoIDQFYJOa0&t=81s
2. Not difficult. Small mods to current cross-member maybe.
3. True. Quick job for a specialist.
4. True but what you have should match unless you have a box/ J OD from a 140.
5.Totally untrue unless you want to use a 240 gearbox and engine and use it upright. Cam oiling maybe an issue if you do. Take this issue out of the equation.
6.Wiring is easy but you may have to drill the cast in boss on the gearbox lid to fit the lock-out switch.
7. OD with 4.10 is good. Much better and quieter cruising. It will drive the same as a 4 speed and you will then have the benefit of the OD. In flat Holland you will be able to use the OD as a normal 5th gear. PV is a bit lighter than an Amazon and it works fine in one of those. I've had one and I'd be happy to have a 4.10 in my Amazon Estate instead of the 4.56.
8. Gear lever doesn't change at all. As above, the top plate does need to have the lockout switch fitted. Some M40 lids come with a brass plug. Remove and fit switch.