So first of all you could just go crusading by asking your lord to go to the Holy lands that way you can plunder your way through the heathens.As far as i know the diplomacy mod is in the mod so you should just be able to ask your king to go to war with faction x because of x reasons you have to have good relations with him though to accept.
And as to your question about crusading you will join the "Holy Lands" faction(dont know the name atm) and while you do lose your fiefs for the time you will regain them once you come back.I dont think the Crusades really ever end except for when the faction that youre fighting against is just gone.You can easily return to your kingdom by just telling your lord that you wish to come back and then you will regain your fiefs.
The pros to crusading are very simple you can plunder the heathens and fill up your moneybags,but there's one HUGE con you will not gain any income from your "old" fiefs meaning that you will only gain money from plundering(i dont know if you can get fiefs when you crusade i at least never got one) another pro is that you can recruit knights of Holy Orders which are quite strong.
Hope i didnt forget anything but if i did here's a link to the FAQ from which i took this informations.
http://forums.taleworlds.com/index.php?topic=231208.0
Oh and i forgot you will gain positive relations after you went crusading with some lords and factions.And those holy order knights can be recruited in a monastery which you can build for 10k in your fortified manor,these knights while expensive are very powerful.