Question:

One site says citizens of some European countries get a visa on arrival at the airport for 30 days. If that's the case I would use my Finnish passport to apply for 30 days.

Answer:

For VOA you should fly to Iran and get a 30-day on arrival visa at the airport. This kind of visa is issued for 98 nationalities excluding USA, England, Canada, Colombia, Pakistan, Afghanistan, Iraq, Jordan, Bangladesh, and Somalia. If your nationality is among the countries authorized to get it, you can collect your visa against offering your passport, a passport-size photo and about 10-150€(depends on the nationality)to the staff at the airport. There is no need to fill in any form beforehand.
You can also fill in the visa form on Iran visa application , it takes about 3-7 working days to get your reference number and two working days to collect your visa at the embassy by direct referral or one working week by posting the docs to the embassy. You can pay the visa fee in cash in Iran.

Check the cheapest Iran tours.