在我个人看来,它将花费很多done well it is an expense that will never have to be done again whilst you live there. Always make sure the company has full public liability insurance. Make sure you get a small brief break down in materials and labour charges. Do not over panic. Find a quality trades man and get them I.E view portfolios, other works, and recommendations. Once you have found all this chill out this big job should be made simple by the tradesman you choose. It’s a huge job huge expense but once vetted and verified a professional tradesman should be able to carry out your job whilst keeping the roof from leaking whilst doing the job. They may use a tin roof or use the old school methods of felting and battening from the bottom up to prevent rain coming in either way. The only thing you have to worry about is picking a tradesman and the job is then for him to worry and think about.

The roof would need to be accessed to get a proper view of things.
If the roof timbers are in good shape then just treat it as a normal re-roof.
If the timber cannot be treated and need to be replaced then you may have to consider have a temporary for erected over the roof so the work can be carried out with the minimal of weathering.
As stated a survey of the roof would need to be carried out to obtain the full picture.
To which I would happily oblige.
References and pictures are fully available.

all re roofs now require building regulations to sign of the work,
your best bet may be giving your local building inspector a call and asking there advice on what materials are best for your particular roof & take from there , most building inspectors i have found to be very friendly if kept in the loop.

good luck Alex