With respect I think you may be a little out of date with property valuation data when comparing Devonport with the Marsden/Bream Bay area.
Marsden Cove is located around a marina/canal facility developed by the Hopper family and targeted at the premium end of the market (sections from $800K - $1.4M).
There is a good supply of more affordable, existing housing towards One Tree Point and slightly further afield towards Ruakaka. There is also a large area of greenfield sites within a 5-6km radius of Northport.
In comparison, I note that the average price for a 3-4 bedroom Devonport property is now @ $2.2m.
Houses and properties for sale in Devonport | Trade Me Property
I don't know the going rate of of a naval base in the same locale, but I suspect it may be more than you think. The proceeds from the sale of these high value defense assets would certainly go a long way towards the cost of new infrastructure in a lower cost, industrial/commercial zoned area.
In regards to support services, there are already a number of local providers with a proven history of working with the Navy (e.g. MacKay, Culham Engineering, NZ Ship Repair, Circa Marine Engineering). Existing providers (and their staff) will choose to relocate (or not) depending on the viability of such a move and their individual circumstances.
I'm not sure retaining the Base at Devonport to ensure ongoing local employment should be the prime consideration.
I agree with your comments in regards to the relocation of Ports of Auckland to Marsden Point (though it is now only @ 2 hours travel by road) However, I can see both Tauranga and Northport organically growing there share of the trade, over time, as POA becomes more constrained. An interview with the outgoing Ports of Tauranga CEO for those interested in this subject:
Shipping out: leading positive change at the Port of Tauranga | RNZ