Your external exchanger would require that the oil always be warm enough to actually circulate it to the exterior exchanger.  The advantage of internal coils is that the oil can get cold in place and be warmed in place when you need it.

Exterior exchangers would require ALWAYS running at cool temperatures, maybe a 99.99 Fail-Safe design, which would probably mean 2 standby units.  If the oil ever set up in the tank it would be a hell of a job getting it moving to the external exchangers again.  

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Good point, but probably would have more than 1 unit to service the tank anyway.  Don't think I could get away with only one since that is a LOT of oil to heat.
With internal coils, many tank penetrations are required (depending on the # of coils required, maybye 20-30), so I'm trying to compare the cost of retrofitting existing tanks with coils (multiple nozzles) versus external exchanger(s),which would require only a few new nozzles.

You can some ideas from the following site and similar:

However many units you need for minimum heat requirements, multiply them by the standby/outage risk increase factor.  

With an internal system, you could shut it down and let the oil set up until needed.  With an external system, it must be turned on well before cool weather approaches and left on until ambient temperatures are not in danger of falling again.  Sounds energy and maintenance intensive.  Be careful with a system like that.  Could turn into a white elephant.

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