OC
NDBs can play games with you; they are affected by terrain, water, weather. They are also damn hard to track when the ADF is mounted 3 feet away from the DI (which is often the case in training planes) which makes transposing the ADF reading onto the DI very difficult
An RMI is the great solution for this, at about £3000+ but that just makes it all the more obvious how inaccurate the ADF can be.
For an IFR departure you don't need an ICAO flight plan; you just ask for it. The UK is unique in this I think; elsewhere it would probably mean an airways departure (SID) with a corresponding flight plan.
In practice, if the conditions aren't obviously atrocious, a lot of people depart "VFR" even if they enter IMC soon afterwards. The transition between VFR and IFR is something you can do as you wish - again the UK is very good in this respect.
I reckon 20-30 hours (ON TYPE) in IMC or poor VMC are needed to keep current enough to be able to fly any IAP as needed. Which means perhaps 50-100 hours a year total time.