[RPG] the minimum craft DC for a wand

It is reasonable that this character can 'take 10' for a roll

Yes you can always take 10 when not in combat or your life is not in danger.

Are all of these calculations correct?

They seem to be in order.

Does a +4 or +6 item just costing more money and time? (same CL)

In the case of belts / headbands, this seems to be the case. Note that Weapons and Armor have different rules (3x modifier).

Can I use a scroll? Can I use a Wizard friend?

Take a look at the Cooperative Crafting text here. The basic answer is yes.

The details are:

1. They must be present for the whole time.
2. They are casting that spell each day during the process.
3. If they are an NPC, they are charging each day.

Clearly if you wanted to do this with scrolls, you could, but you would need to expend the scroll every day you are crafting. At 150gp * 4 days, that's pretty expensive.

Does that make the earliest level for a non caster 7 to craft magical items?

• There are classes with odd bonus feat progressions such that you could get two feats at level 5 and spend them both. For example, the Psychic Warrior has this.
• I guess you could also "not take" a feat at 3rd level and then spend them both at 5th level. I'm not sure if there are explicit prohibitions against this.
• There's also the concept of "re-training" of feats as allowed by some DMs (see comments)
• In general 7th level is the minimum for magical crafting without magical crafting abilities.

I'm afraid not. Since you are required to be a 5th level Wizard to cast a Fireball spell, it must be crafted as a CL5 wand. You can go higher, and make a CL10 Fireball wand, but you cannot go below CL5, as that's the minimum level required to cast the spell.

From the following document: http://paizo.com/prd/magicItems/magicItemCreation.html

Magic Item Creation

While item creation costs are handled in detail below, note that normally the two primary factors are the caster level of the creator and the level of the spell or spells put into the item. A creator can create an item at a lower caster level than her own, but never lower than the minimum level needed to cast the needed spell. Using metamagic feats, a caster can place spells in items at a higher level than normal.

Craft Wand

The creator must have prepared the spell to be stored (or must know the spell, in the case of a sorcerer or bard) and must provide any focuses the spell requires. Fifty of each needed material component are required (one for each charge). Material components are consumed when work begins, but focuses are not. A focus used in creating a wand can be reused. The act of working on the wand triggers the prepared spell, making it unavailable for casting during each day devoted to the wand's creation. (That is, that spell slot is expended from the caster's currently prepared spells, just as if it had been cast.)

Also, you must be CL5 to take the feat "Craft Wand"

From http://www.d20pfsrd.com/feats/item-creation-feats/craft-wand-item-creation---final

Craft Wand (Item Creation)

You can create magic wands.

Prerequisite: Caster level 5th.

Benefit: You can create a wand of any 4th-level or lower spell that you know. Crafting a wand takes 1 day for each 1,000 gp in its base price. To craft a wand, you must use up raw materials costing half of this base price. A newly created wand has 50 charges.

However, if you're looking for a low level damage spell and you want to make a wand at a lower Caster level, you might want to give Burning hands or Scorching Ray a shot.