Without knowing how many layers or whether or not it’s wet, it’s hard to advise. But I’ll try.
Are their any leaks?..if no, than can install coating every 3-5 years to try an extend the life. Remember, coating is to extend the life, not to prevent water from getting in. Figure around $1.50 a s.f. to do so.
If the existing roof insulation is not wet and no multiple layers, then you could do a cheap modified overlay and get 10-12 yrs out of it for around $2.00 a sq ft.
If it needs tore off and a good 15-20 yr system installed such as epdm(rubber), I’d say around $5.00-$7.00 a sq ft. Tapered insulation(if the exisiting roof is too flat and holds water), is needed, then you could have an additional $1.80-$2.50 per sq ft.
Make sure to get advice from a commercial type contractor. They will have more experience with long lasting flat systems. Ask what type of work they do, percentage wise. If they do mostly shingles, chances are, their expert and workforce knowledge will be on shingles. A commercial contractor that does some residential would be preferred. They’ll have the commercial knowledge and still know how to go easy on the shrubs and bushes.
From the pictures, the roof is not in terrible condition. However, appears to be getting toward the end of it’s life. Coating may help prolong as discussed but there are to many variables that determine exactly when a roofing will start to fail. Weather, winters, ponding water, and building movement to name a few. If you start to get leaks in the middle of the field of the roof(and it’s not a puncture), it’s most likely ready for replacement. If leaks are near walls or pipes, it’s possible that these can be patch and still last a while.
Hope this helps.