There is absolutely nothing wrong with the use of 
a flail mower or flail chopper for this instance.
A used municipal mower with three flail mowers 
would also be a very economical option to handle 
this work and offer him up to 24 FEET of cutting width
as a possible and very relevent option if he is willing to 
spend time looking at municipal auctions where they 
are often found in good condition with low hours due to 
seasonal use by the municipality. 
The mower parts are readily available and economical 
in cost to purchase for repair and replacement of these 
municipal mower tractors and are typically Alamos less
than 15 years old and are easily repaired and maintained 
by someone who may have little experience with machinery. 
The tractors also have easliy obtainable parts and authorised 
brand dealers and this is important as well; The used 
municipal tractor has been well maintained and may have a
lot of hours at first glance BUT it will have a long service life 
for 10000 hours or more if maintained. 
The 12-20 foot or wider flail choppers are every well 
built machines that have square tube frames the entire 
width of the machine to maintain full width strength for 
the rotor,  bearings, and tires.  
The flail chopper is a mainstay for many row crops 
especially corn,  cotton,  tomato's and peanuts.
they are also uysed to cut hay ground back to the 
ground before winter to reduce hay sod damage by 
many farmers.
The flail chopper is easy to transport with wheels sets 
that are lowered and allow it to be towed from the end of
the flail chopper making for a very narrow implement 
of less than 6 feet in towing width.
 
The flail chopper makes quick work of shredding crops
with little work using very high rotor speeds to cut 
quickly and cleanly.
The flail choppers knives are also very low in replacement 
cost versus a single rotary cutter blade per foot of cut or 
slice in a flail shredders case
My desire in discussing the flailmower option simply 
comes from my 35 years of using them,  maintaining them,
and how well they can keep down and kill invasives rather
than other methods using herbicides as they have a greater 
in total length cutting edge surface length versus the typical
 rotary cutter or bat wing mower with the flail shredder having 
the greatest cuting edge length of all of them.
The absolute fastest way to destroy brush and invasives is with 
the weed dragon propane burner which can kill and knock down 
and kill tall weeds with no issues for the user and it will directly
kill and knock down most standing invasives as they are the ones 
which are growing faster and rise above the grasses in most areas 
that someone wishes to keep under some form of grassland 
management or weed control. 
A used wide cut windrower would also work as the brush is simply dumped 
and which could be used as a compost crop if desired. 
GEE WHIZZ,
I could have said his life would be even simpler if he 
spent the same money on a good 8 foot sickle bar 
from Ken Sweet one of our fine supporting sponsors in good 
standing or simply purchased reusable weed barrier fabric from 
www.Farmtec.com
for the entire acreage moving it from plot to plot, kill it entirely,
avoid mowing the year round, and have good crop land in few years.
BUT I DID NOT!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!