if you have a straight shaft machine then steel heads are already
availablefor the better models, so you can switch from line to steel
and back at will, unless yo are looking to having 2 machines? it is
better to work 1 machine until it drops you get the best out of it
depending on thickness of undergrowth you may need to use the saw
blade head? can be difficult to use and when using steel never have
others around they can be very dangerous when and if they kick back, a
good practise when using steel is to keep your body behind the machine
so if it kicks back you can resist the thrust of the kick back, hope i
explained that ok very hard to put into words.
also sounds like you might need to look at machines at or bigger than
say 27cc and of course then using handle bars and a harness for lots
of work could most likley be the go. these machines do have
limitations and in some instances may not do as much work as a brush
hook would do.
With peace and brightest of blessings,
"Be Content With What You Have And
May You Find Serenity and Tranquillity In
A World That You May Not Understand."