http://www.hse.gov.uk/pubns/geis5.htm have a look at this and see what precautions you will have to put in place. This will help you get an idea of price... :)

If you are a company in the solar panel installation business, I am surprised that you do not survey all roofs for structural integrity as part of the pre-installation assessment. Presumably you know how much weight per surface area your panels apply, with additional factors for high winds etc. This is not a safety matter (yet) but a viability one, for which a surveyor or engineer is required. Most metal barn roofs are low strength and as they age, they deteriorate. Eventually they hole - in less than maybe 20 years. It is not just the panels, but also what is holding them up. In my experience could be widespan wooden frame, although steel is used now. How the panels are attached to the frame is another aspect - solar panels are surely going to create 'lift' pressure in winds? I doubt you will find it feasible to install solar panels on such a roof, without additional strengthening. So until THAT is costed, waste of time even thinking about safety systems for installing it.

Oh for an edit function, slight mistake in terminology. When I have used the term "installer" I am referring to the complete "entity" the actual physical "body" doing the install, may well not be competent or called upon to do the assessment of the roof, more often than not in my experience they are hammered down on price and are expected to perform miracles for peanuts. Also, they are often not actually sufficiently competent to do the install with the information they are provided with, as it is rare for the to have a design to work from and they are more often than not not competent to do the design. This is a perfect example of an industry maximising profit at the expense of competence, and I have seen it with my own eyes. I've even taught guys who were quite full of themselves with their methods until they had the potentially lethal error of their ways explained, and then they were enlightened to the issues they were creating. The worrying thing is that only one out of dozens was even concerned that their work could be lethal or cause fires!

Aged metal barn roofs are not a great deal safer than corrugated cement panel roofs though there is a perception that they are not so fragile and so people work on them without precautions only to find the roof collapsing beneath them. They deteriorate due to wind and rain causing rust, failing bolts and fittings, lifting panels, loosened cross-members, etc. A York solar panel business took no precautions at all when working on a fragile roof and ignored the advice of the farmer to take precautions... http://www.hse.gov.uk/press/2013/rnn-yh-9813.htm

Just working in a farm environment has enough hazards, from slurry tanks to livestock, let alone roof access on what can be very unpredictable material. I think a professional engineer is required to determine swl etc to decide if such a project is even possible and if so what additional strengthening work is required. Bob