I'm not sure about the other questions you've posed, as I have not played RTW or M2TW. However, I can try to comment on a good beginning faction.

I suggest staying away from Muslim factions for your first play through. They require a different style of play on the battle map, and playing them like a Christian faction will result in you struggling - the exception is the Almohads, which are able to adapt to such a playing style quite well.

I also argue against choosing a Catholic faction as you may find that the Pope will get on your nerves if you invade your Catholic neighbours, which you are bound to do at some point. Of course doing a hundred turns or so with a faction like the Danes or English, who have bordering territories that are under rebel control makes things a little easier. Even the Poles, although they are not the easiest faction to start with.

So, who does that leave? The Byzantines! Here is my reasoning:

1) Orthodox, so you don't have to deal with the Pope.
2) Lots of armour, so you can play them like you would a Catholic faction.
3) Excellent units in the early period.