Yes, you can change your secondary, but only when you get to a certain point in the game. In Factions and Nightfall it's fairly early in the game, while in Prophecies its a way off. Once you unlock those professions, you can change them whenever in a town or outpost.
I don't really understand what you're asking in your second question. If you are a healing monk, you'll want a staff with a healing req as with all other attributes. Also, if you're healing on a necro (or using any healing spells at all), don't.
The 2ndry profession changer is in one of the towns near the beginning. I am not sure where exactly. Check guildwiki.org and search the NPC's. It does cost you some gold to do it.
Talk to Senji of Senji's Corner. You have to beat the Nahpui Quarter mission to access Senji's Corner, however. Changing your profession costs 500 gold (you can switch back to a secondary profession for free after purchasing it though).
You can only heal from spells or recovering after combat. Monks are very useful in that regard, though as a Necromancer you could use life-stealing.