Allison auto on downhill run

Hi all, looking for clarification on procedure as I'm planning to buy a 315HP 8x4 with an Alison auto rather than a manual. Got my license in a 18sp RR Kenworth, and that was a good lesson in what's not of me. Yep happy to be woose, but I plan to spend my time better behind the wheel than working the stick. The job requires moving about 16.5T plus truck (25,000kg GVM) down a windy hill about 8% and then come back up empty. My driving instructor told me knock it down early and make sure your life insurance is current. The Alison service agent said the optional retarder is not required, which suits me as its quite a bit to have it optioned on, but I'm also not against adding it either.
Not a huge load but still if it gets away there is not much road to get it back in line.
So just checking if the consensus on the rule of thumb is,

going down use one gear lower then the one used on the way up if loaded,
get it in that gear well before going over the edge,
and don't rush it, if the run takes x to complete then enjoy the ride at x

Thoughts

 

Thanks I presume in a manual its a bit the same process, and what I'm looking at is the same purchase price as the manual. I was thinking as been suggested if the retarder reduces brake maintenance there is a downtime and cost saving element founded in the initial outlay. It would add 11.5% ($8K) to up front cost though.

So I guess next question would you do it on a second hand truck?

I'm looking at an ex cement truck, so its done a few miles on the motor, but just run in the componentry. I have a seller near by who cleans them up with new brakes, air bellows, removes rust, and paints them. Gets them serviced and fixes what needs done. They come out a tidy unit at the end. 280K km and 14500hr for a 10 year old truck supposedly x owner driver cost $70K. This is also a common price from a private sale with nothing done. Have spoken to Cummings about what a motor equal to 1 000 000km would need, and that seems easy enough and adds 14% ($10K) to the purchase price. I guess by not getting Cummins to work on it that will come down. I don't see much point getting a start up going and then find out I'm in the work shop letting customers down, so for me the motor freshen up is a given before it starts rolling. The plan is to put another 800 to 1 000 000km on it over the next 8 to 10 years. So I think I have just answered my question on the retarder. Do you think I'm being unrealistic with service life or is there a better way.
The work will be reliable and constant, just can't afford a new truck from the start.