A wizard is going to have an extremely high Int. This influences your answers:
1) Your base reflex save will be very bad. People building evasion wizards take the "Insightful Reflexes" feat. If you take this, your reflex save will be plenty good enough for evasion. In fact, this is the primary reason people build 18/2 splash wizards.
2) Rogue is the standard splash. Both Rogue 2 and Monk 2 will give you evasion. Monk will give you better AC (worthless, since your AC will be nonexistent anyway), stances (none very helpful to a wiz), a free feat (toughness is useful!), and better unarmed fighting (useless on a wizard, at least after the first couple of levels). Rogue will give you sneak attack (useless), a few weapon proficiencies and light armor proficiency (useless, at least after the first couple of levels), and rogue skills. Rogue skills are very useful to soloers, and a warforged arcane is an excellent soloer. A warforged wiz 18/rogue 2 is almost completely self-sufficient. Wizards, again because of that high Int, can take full advantage of these skills. It's possible for a properly geared wiz 18/rogue 2 to handle any trap or lock in the game. Most importantly, you will be able to reach a high UMD score.
3) A pure wizard will have higher DCs at end game. They will be more effective in epics, especially. A rogue/wizard will be more survivable, easier to level, and a better soloer. It's really up to you -- there are advantages to each. I don't personally think a monk/wizard brings enough to the table to be worthwhile.
4) For a pure wizard, a recent example build is
here. It's worth reading the whole thread to get a sense of people's suggestions and why the build ended up like it did. The "classic" wiz/rogue is SigTrent's
Arcane Archeologist, but it was made pre-Archmage. A more recent thread, by the same author as the first one, can be found
here -- again, read the whole thing to get an idea of the tradeoffs and see other peoples' suggestions, as it's not perfect. In particular, that build would be better with a true neutral alignment.