Before anyone can give you a godd "Guesstimate". They would need to know more information;
1.) what type of tires/wheels are on the trailers & material make up - rubber, polyurethan, steel, etc.
2. What is the operating surface - smooth concrete, brushed concrete, wood block, etc
3.) Any grades/ramps to be negotiated.
4.) What is the percent of grade (divide vertical height by horizontal run)? And how long are they (grades/ramps)? Average number of trips up the grade per work shift. Your Stihl dealer representative should be able to help you - if not contact Stihl directly via the internet you might be able to find your local representative & contact info.
Hope this helps!