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So here are the teams:

The Horsemen of Apocalypse:

-Mikhail Rasputin (Matter manipulator, superhuman strength and endurance)
-Abyss (Strands to ensnare victims, telepathy, strength drain)
-Holocaust (Superhuman strength, invulnerability, microwave blasts, lifeforce drain)
-Mister Sinister (Cellular-control over his body, regeneration, telekinesis, telepathy, injury resistance, immortal)

Avengers:

-Ironman (Armors have an array of weapons, superhuman strength, superhuman fortitude, flight)
-Quicksilver (Superspeed)
-Hawkeye (An array of special arrows)
-Vision (Ability to change his density)
-Beast (Enhanced agility, strength and endurance)
-Professor Hulk (Superhuman strength, durability and healing factor)
-Scarlet Witch (Ability to effect the probabilities)

And here are the scenarios:
-1. No prep at all
-2. Avengers have 1 day of prep, Horsemen have none
-3. Both have 1 day
-4. One week for the Avengers, 3 days for Horsemen

Personally I would put my moneys on The Horsemen of Apocalypse, although Avengers do have a chance in every scenario, especially in number 2. Avengers are going to have rough time with Holocaust and Mister Sinister...

The Horsemen take one and three, Avengers two and the fourth one could go either way. Without Ironman, the Avengers would stand no chance.

And Thor was intentionally left out. Avengers still, however, have chances of winning because of Ironman as you mentioned.

When Avengers have prep they are allowed to use the resources of Anthony Stark, but lest the match go unfair they are not allowed to call reinforcements.

The Horsemen also have resources and Mister Sinister sure knows how to plan fights.

Here goes my version of this fight. The feud is based on the scenario 2.

The day of prep of course partially goes to studying their enemies since they really don't know anything about each other. And following equipment would be purchased (also includes some general preparations) :
-Ironman would get his Hulkbuster armor and would install all the information of the enemies to computer system of his armor.
-Vision would reload his batteries so that he will be able to use his maximum capacity and he would also download all the information of the enemies to his brains.
-Beast would just study his enemies and do some normal exercise and revise fighting strategies.
-Hawkeye would take some arrows causing sonic disruption in addition to his normal arrows.
-Hulk would keep himself ready and motivated just like Scarlet Witch and Quicksilver.

And then to the battle itself:

Vision puts his density down and goes to fight Mikhail Rasputin by partially solidifying inside him and trading blows whenever possible.

Scarlet Witch would use her chaos bolts to hurt and disturb Abyss while Hawkeye would use his extremely effective sonic disruption arrows on him (At least Banshee managed to cause serious damage to Abyss via sonic disruption).

Ironman would fight Holocaust by dodging his attacks and weakening the armor.

Meanwhile Beast and Quicksilver would keep Mister Sinister busy by constant attacks so that he cannot concentrate (After all this is a surprise attack since the Horsemen are not aware of the situation before they see the Avengers).

Soon Vision would kill or defeat Mikhail and go help on destroying Holcaust's armor. When that is done Ironman would simply scatter Holocaust's energy form all over the world just like he did to Living Laser.

After this Ironman would go fight Sinister and Vision would go destroy Abyss. The remaining members of The Horsemen of Apocalypse wouldn't be able to win at that point...

Of course that was just one possible course of action, please post yours.