I'm about to leave Oman to drive my Land Rover to Britain.I expect to go through Iran, Turkey, Bulgaria, Romania, Moldova, Ukraine,
Poland, Germany, Belgium and France.

Re. the green card, is it likely to be more economical to buy it in advance, for three months, or pay each time I'm asked to, at borders?

And paying at borders, is there one I'd cross where the insurance I was buying would cover me for all the European countries ahead?

If I were you I would try to get a Green Card in Iran, because Iran is part of the Green Card circuit, accepts foreign green cards and (I guess) should also seell them.

However, once you have found an insurance company that releases the Green Card, you have another big issue: you must check whether the Green Card of that company includes the coverage for all the countries you want to cross.
This is because the Green Card shows a table that lists all the member countries (e.g., all the European plus Tunisia, Morocco, Russia, Iran, Turkey, Israel, ...), but often some of them are "cancelled" by the company, because the company does not want to cover you on that countries.

This is an example: suppose that you get an insurance with complementary Green Card in Iran. You cross Iran and everything is fine.
However, once you are at the Turkish border (Bazargan-Gurbulak, I suppose), the Turkish officer finds that the "TR" cell in your Green Card has a black "X" on it. That means that you are not covered in Turkey, and you will have to buy a Turkish temporary insurance (and I don't know whether this is possible in Gurbulak).

To conclude, the problem for you is not only to decide whether to get or not the Green Card in advance but also whether the Green Card you are getting actually provides you the coverage in all the countries that you intend to cross.

Let me know if anythins is unclear.
Regards

Hi, i would advise buying a Green Card if possible, it will probably save you money and will definately same you time and hassle at the borders, Andy

cruiser81 is correct, and incomplete.

Even though Iran is a "signatory" to the Euro Green Card protocol, I'd be very surprised that you could purchase a Euro Green Card policy from an agent in Iran, or at the Iran border entry, that includes any onward countries beyond Iran, e.g. Turkey, etc.

And no, paying at borders, there is no one border where you will be able to buy Euro Green Card policy for all the Euro countries ahead of the specific country you're entering.

It is possible to buy--in advance, before you leave Oman--one Euro Green Card that will cover all EU countries: including Bulgaria, Romania, Poland, Germany, Belgium, France, UK. This will cost you between ~200 Euros + per month of coverage (for a Land Rover), depending on the agent you purchase from:
Green Card
ARISA Assurances S.A.
How does the European Green Card Insurance work?
TourInsure (may also include Turkey in Green Card?)

But you will still have to buy insurance at the border entering Moldova and Ukraine.

Once entering/ inside the EU, it is also difficult to find insurance agents at each border that will sell Green Card. But it's expected/ required in case of accident or document check. Better to get the EU part in advance