I cannot speak for the SK-5208 as I do not have experience with those units, but a catastrophic (especially one which is sudden) failure of a 4005 is very unlikely. The panel will definitely show signs of failure before it completely quits. The 4005 is one of the most reliable (“modern”) panels that Simplex has ever produced, and I have heard that the SK-5208 is also quite reliable.
Parts for both panels are still available on sites like eBay, but you may have trouble finding something brand new - used parts are still available quite widely, but I do not know if the fire code where you are located permits used parts to be installed in a system.
If you do plan to upgrade the buildings at some point, you may want to opt for a 4010ES instead of a 4007ES, as the 4010ES is generally considered to be more reliable (unfortunately, the 4007ES has a bad track record as of recent, mostly due to quality control issues on JCI’s end).
EDIT: Depending on the size of the 4005 and 5208 which are currently in operation, you may be able to opt for the 4006 instead. It’s the last conventional system that Simplex is currently making, and can be expanded up to 10 zones. If you need more than 10 zones or would like other features that the 4006 can’t provide (like Priority 2 alarm or similar), you’ll probably want to upgrade to addressable.
If you decide to install something besides Simplex, I can’t really help out there as I don’t have experience with many of the newer systems from different brands.