Well we are in our early 50's and are about to take off ( on Sept 2nd) for a 2 1/2 year stay in Europe ( then we will go on to other continents for a very slow trip around the world) so I have learned a few things & will probably learn much more as we go.I will do a blog that might help you,but I have yet to get it up.
Its not really that easy to do such a long trip and first you must decide how you will handle the shwengen problem as it will get worse in the near future ( more countries joining it ) & also the problem of the falling dollar ( which also will probably get worse in the near future).
I have lived in Europe for a year before ( Italy ) and suggest you read lots of the complaints of expats who live in Europe because you will run into that culture shock and you might as well be prepared and ready to deal with that as it comes with long term extensive travel.Perhaps you have lived abroad before and already know this reality.
Every long term traveler must follow what works for them as there will be different reasons to pick different areas.Languages will impact ,especially when living in an area ,so certainly that will color a choice.Also weather preferences as some areas of Europe have much worse weather than others ,especially in the winter.We are also traveling with our 5yo ( who will be 6 in the fall) daughter so that will impact us.Also ofcourse budget matters and big cities will tend to be more than rural areas,altho suburban areas just outside cities can save one lots of money sometimes.
We have decided to have a winter base in rural Andalucian Spain near ocean and mountains ( not too different in weather & topography to our current location in northern California as thats what appeals to us).We will return there for 2 or 3 winters for 5m each and do short trips to places in Spain,Portugal,Morocco,Egypt and an African Safari from that base.That way we can get to know authentic life in Spain living in a 16 century village of 2000 people, my dd can experience the culture thru the school,and it can become our home away from home.My husband and child are fluent bilinguals in Spanish ,so thats one reason for picking it as our base ( tho we might due italy ,France or ? 3rd winter).The nice thing about an open ended trip ,one can always shift plans.
Unlike you ,we will have a vehicle ,actually we are going to buy 2 different RV's while there as its such an easy way to travel for us.We will follow the weather basically.Croatia is beautiful ,but I don't want to spend my winters there.I only like cold weather when I am skiiing
and even then I have been spoiled by Tshirt skiing in powder here.
Look at whats available rent wise in areas that interest you.I was amazed at all that I found available for the winter in Spain.If you go off season ,you can get some great deals for long term rents.We have a spectacular 4 bedroom home that has ocean and mountain veiws that goes for a pretty penny at a weekly rate during high season.I contacted them on a lark because I loved its looks even tho it was way over my budget and I ended up getting it for exactly my budget since I booked it off season for a lengthy stay.
The cost of living is high in most of Europe ,so you will have to take that into consideration in your choices.If all your travel will be by train ,then you will have to pack really lite,so take that into consideration as well.
The prospect of Europe for 2 or 3 years is too big a question really,so you will have to break it down and what you want will color your choices.We are using a long stay visa ,its much harder without one ( to do it legally as we are only allowed 3months out of 180 in schengen countries which is most of Europe..so if you are 3m in ,you then must be 3m out).Ofcourse its also very hard to get the long stay visa usually as well.
You can learn a lot by people who have spent long term stays there.Its not very common,but those who have done it are full of great info ( like Kaydee as someone else advised).I have also learned a lot from people who are "permanent travelers" who have been on the road for years and have it as a lifestyle.
I have been studying hard for a year for this trip and I have barely scratched the surface on much of it.I will need to keep reseaching as we go.It helped me to get an overall plan and focus on the first parts of the trip.( We are coming into Amsterdam because thats the only place you can buy a camper and then making our way to primarily parts of France and Northern Spain before getting to our rental on Nov 1st).Just France and Northern Spain is endless on information gathering,so I have only done small amts on ohter areas & will focus on them once we get into our rental.
We will go to Croatia,Budapest,Prague etc Eastern Europe the first summer ( we will fly into Moscow & St Petersburg as well for a short visit from there) and Italy ,Greece and Turkey our first Spring ,so hopefully my blog will give you some info that might help.Read all the trip reports here and elsewhere on areas that interest you.
But you will be pioneering in a way as not too many people do such a trip.Its not quite an expat and its not quite a traveler.You can learn good info from expat sites and full timing RV sites as well.
Good luck! Its a great adventure and I am sure you will have a ball and find the perfect places for you!
Slow Travel Talk 
