I'm finally ready to purchase DLCs, but I need some advice from the people who have already played them:

1) Which DLC should I buy first, and in which order should I buy them if I buy more than one?

2) How well do they "stack"? Do they cause each other to glitch? Is it better to install and play one, THEN buy another, or is it ok to install several at once?

3) What level should I be before I enter DLCs? I understand that level 20 is recommended for Dead Money, but what about Honest Hearts and OWB?

4) Are the perks and traits from the DLCs only available after you enter them, or from the beginning of the "vanilla" game?

I'd say in order of release: DM, HH, OWB. I see no reason to do it another way around.
Perfectly.
I'd say 30, the vanila level cap.
Right off the bat.

I couldn't be bothered to separate my answers into points even though it probably would have been less effort than typing out this sentence.

1. I would buy Dead Money first, because there is an intertwining story linking the DLCs and it's good to go through them chronologically, though they can also be played out of order with no problems.
2. The patches for the game up to this point have fixed bugs and other issues, but in addition they added DLC support. So they stack very well in my experience. Though I am not certain about downloading DLCs on the PC, but I believe that it should work.
3. I think that it is 20 for Honest Hearts and 15 for OWB. For OWB I would suggest getting high levels in Guns, Speech, Energy Weapons, Medicine, and Science.
4. The perks and traits from DLCs are available at the beginning of the game.


Hope this helps!

No oddness from doing them all. I mean, these are offical add-ons not mods.

Wait a minute, Guns AND energy weapons? I mean, I realize that OWB adds a lot of energy weapons and has a lot of robots, but are two combat skills really a must?

Also, would a Cowboy guns or shotgun-oriented character fare well in the DLCs? Do shotguns or cowboy guns spawn in Dead Money and OWB? (I know they spawn in HH).

Yeah well, the main game was official too, and I discover a new oddness in every play session.