My experienses on 2 meters when I was living in southern Finland (near Helsinki). My Finnisf call sign was (and still is) OH2AXE. I did not have 70 cm equipment.
Antenna: 2 x 2 stack of double hybrid quads, gain about +12 dBd, about 10 m above ground. 50 W homemade linear amplifier.
Most of the time the TX power is not so critical, even 5 or 10 W is enough to work DX. The limiting factor is your ability to hear the DX stations and for that a higher gain antenna helps a lot. I seems to me that +10 dBd (over dipole) is the minimum for DX. At the same time it increases your ERP (Effective Radiated Power) for TX by the gain (in dB) given for the antenna.
FM contacts up to about 250 km away were always possible in more or less any direction.
SSB contacts via sporadic E to Yugoslavia (as it was then), England, Poland, Germany, Italy and other parts of Central Europe. To my experience sporadic E is specifically a summer phenomenon, between abou April and September. I have never experienced it during winter. It is somewhat rare on 2 meters, though. But if you start hearing strange stations from your FM (89 - 108 MHz) radio, check the 2 meter situation immediately, the sporadic E may reach even up to 145 MHz.
CW contacts via aurora to Russia (longest to Archangel, Moscow and the Ural mountains), the Baltic states, Sweden, Norway, Denmark, Poland, North Germany. Via aurora only CW is intelligible and it sounds like hiss-hiss-hisssss-hiss ---. There is no tone and the report is given for instance as 59A (A for aurora tone). To my experience radio aurora is mostly a winter phenomenon and in Southern Finland the antenna had to be pointed more or less to North (say from NW to NE) for contacts. Note that most of the time there is no visible aurora, but the radio aurora may still be quite strong.