Option 1, depending on how clean you can get the system is probably a better choice than option 3 despite option 3 having bigger waterways and the ability to split it for cleaning, my experiences of this type (3) of PHE was on a number of ships I sailed on, they needed very careful re assembly with the correct torque settings for the tie bolts etc after cleaning, the rubber gaskets frequently were torn on splitting them and any new gaskets had to be glued on, if you go down this route then who will disassemble, clean and re assemble them properly?.
Option 1, if you are prepared to treat the Hx as a throwaway item should it get plugged is far less trouble IMO but of course may prove very expensive in the long run.
My preference is still option 2.
Of course, as suggested above, by others, you could go for a LLH, with suitable filtration, both magnetic&particle, of the boiler return, you might find that the boiler Hx might last for years and its replacement cost may not exceed and may even be far less than the maintenance cost of either of the PHEs.
