I just wanted to chime in here, being that I think my own personal experience along with the collective experience of many other forum members on the RX7Club and the SupraForums would be very helpful.
1) Read this:
Grundy Insurance for the difference between Agreed Value, Actual Cash Value, and Stated Value policies. You want an Agreed Value policy. Anything other than that will NOT guarantee the payout in the event of a total loss. Period.
2) Don't believe what the regular (non-specialty) insurance companies (State Farm, Allstate, Progressive, etc) claim with respect to providing you with "guaranteed" coverage. Trust me.
Traditional insurance companies DO NOT underwrite Agreed Value policies. No matter what they tell you. Unfortunately, the insurance salesman (that's what they are) will SELL you whatever policy you want (agreed value, cash value, collectible, etc), only for you to find out in the unfortunate event of a claim that the UNDERWRITING Dept (the only guys who matter) don't write guaranteed policies. They'll INSURE up to a desired amount, but will STILL perform a market search and give you "fair market value" for your "used vehicle" based on "depreciation" with age and use, at the time of a claim/loss. What that means is, you've been paying a premium for a level of coverage you never had, and only find out you're hosed when it's too late (the time of a claim payout).
Another similar thing. If you have homeowner's or renter's insurance, it'll cover your personal belongings stored in your house up to a certain amount, right? WRONG. Most traditional home insurance underwriters SPECIFICALLY have clauses EXCLUDING coverage of car/engine parts, even if/when stored in your home.
There are several well-established specialty insurance companies that write nothing BUT Agreed value policies. Companies like American Collectors, American Modern, Hagerty, Grundy, etc. How much they'll cover your car for is based on what you can convince them your car is worth, depending on condition, mods, recent comparative sales, etc. And once you agree on a price, they will GUARANTEE (up to that) amount in the event of a (total) loss. No if's, and's, or but's.
Bottom line, get it in writing, from underwriting. Otherwise, you're throwing your money away. And preferrably, deal with someone who's PRIMARY business is writing such policies. Then you'll have some peace of mind.
3) All the specialty insurance companies will only write Agreed Value policies on pleasure cars. Cars you only use for occasional pleasure, shows, events, etc. Some of them provide a mileage allowance; some of them are unlimited mileage as long as its an approved usage. And some will even allow limited driving to work (eg once a week or once a month etc), but for a premium, of course.
But only one will insure your daily driver as well:
Grundy Insurance (specifically their MVP Program). I got a quote on it, and it was EXPENSIVE, so I passed on it for the M5.
4) Last thing. If you wanna find the BEST Agreed Value policy for your vehicle(s), go through Rally Insurance:
Classic Car Insurance, Auto, Life, Home, Commercial - Illinois. They're a broker for all the specialty insurance companies, and as such, will shop around the different companies based on the type of car and usage you're seeking coverage for, and can provide you with the best rate (cuz they're a broker); cheaper than you can by contacting the Insurance Company directly.
Just my $0.02. Hope it helps