Hello there extopia.
I had a question like this on my site recently.
Long grass and a rotary just don't go together.
You will spend half the day unclogging the mower.
There are two machines however which are suitable.
1. The brushcutter. This is basically a strimmer with a metal cutting blade instead of cord.
Its great on tough grass, ivy and brambles.
It will, of course, sometimes get tangled like a stimmer.
2. The pedestrian finger bar mower. You may have seen a larger version of this machine working on a farm.
The cross-cut blades make short work of long grass, thistles etc.
You just have to keep a watchful eye out for rocks which can damage the bar.
Both of these machines should be available from the larger hire shops.
Using both of these machines in long grass, I suggest cutting from the middle of the site out wards.
This will allow hiding wildlife to escape.
Try and use these machines during a dry day as the grass will be much easier cut.
You will of course have to rake the mowings off to achieve a really clean look, if theat is what you desire.
Hope this helps.