JMHO here …
prep involves pounding with a hammer and then wire brushing.
This will loosen exterior rust flaking, though I can not see them working to expose clean metal on the entire undercarriage.
As far as the internals of the under carriage …
Beating with a hammer will knock loose flaking rust, but you can forget about any wire brushing.
FYI..
If they are using some type of rubberized or asphalt rust proofing on the exterior undercarriage.
It is better than even money it will crack and/or separate allowing moisture between the undercoating and metal.
The vehicle is 9 -10 years old.
My suggestion, if you want it done as good as possible would be to do it yourself.
I would follow the basic outline the TSB suggests with a few changes.
Instead if beating the frame(s) with a hammer.
Invest in a inexpensive air hammer.
It will work better and save time.
Instead if a wire brush, try a assortment of wire wheels on a drill.
Again, it will work better and save time.
After removing and cleaning off as much rust as possible apply a rust converter to the bear metal both internally and externally.
Now as far as rust prevention…
For the internals of the undercarriage, I would suggest CRC Extreme duty Open Gear and Bar Chain Lube
This stuff sticks to metal, and remains ” soft ” , it does not dry out, split or crack.
And it wicks into all areas.
As for external rust proofing.
I use a mixture of boiled linseed oil, bar & chain oil and melted toilet bowl ring wax applied with a paint sprayer.
Keep in mind no matter what, rust proofing will need to be reapplied every year or two no matter if you have it done professionally or yourself.