These are the must have mods
Darn ui
Destruction
DC Interiors
Enhanced Weather + Weather Overhauls (depends on your choice)
And for gameplay overhauls theres fook2 or FWE
But personally i suggest FWE+MMM+EVE+WMK combo,
And after you get into the game and finish the main quest, try out enclave commander, its awesome
Now to answer your question
1. A mod that adds more choices in houses other than the Ten Penny Suite or Megaton.
2. A mod that adds more to being a part of Reily's Ranger or adds another Mercenary Faction.
3. A mod that adds more weapons.
4. A mod that adds more creatures. (Marts Mutant Mod any good?)
1. Browse through http://www.fallout3nexus.com/downloads/cat.php?id=72&page=1 untill you get one that suites you
2. http://www.fallout3nexus.com/downloads/file.php?id=5731 or http://www.fallout3nexus.com/downloads/file.php?id=6068
3. Classic fallout weapons, fook2, 20th century weapons
4. Marts mutant mod