Hello Bazzer,
Welcome to the Forum.
I would argue that, because of where you live, undersealing should be considered essential.
I am lucky enough to live by the sea, all that lovely high humidity salty air !
Your quote seems high
There is a major thread on this Forum regarding insurance; but the market seems to change quite quickly.
Several insurance companies won't insure Alphard / Vellfire models; and if you have it converted the alternatives become even more limited.
Your quote seems high
I doubt I would be comfortable using used parts ( I am sure many members are) but I am lucky as I have a very (Alphard) friendly local Toyota main dealer who, unusually, is very willing to work on my machine.
This is a key point; unless you are willing and able, finding a competent mechanic can be more problematic that finding the parts.
Great machines, especially if you buy wisely. Good Luck.