How much does it cost?

From DMG p135:

Uncommon Magical Items are valued between 101-500gp

also:

"unless you decide your campaign works otherwise, most magic items are so rare that they aren't available for purchase... ...In a large city with an academy of magic or a major temple, buying and selling magic items might be possible , at your discretion..."

My understanding is that using an item like this does not count as an attack, is that correct?

My understanding is that using an item like this does not count as an attack, is that correct?

 Correct. 

Edited: I was corrected below.

Some magic items and other special objects always require an action to use, as stated in their descriptions.

What sorts of classes can use this? Any magical class? Or only magical classes that can learn magic missile?

The wand itself has no restriction on what class can use it, so any class can be it barbarian, rouge, or a caster who know the spell magic missile.

Of course your DM might add stipulation of use outside of what is usually required by the item, be it adding a class, level, or ability restriction. In the case of wands command words were common in older additions  of the game

Are you sure about interacting with an object for using the wand?

Found this:

If an item requires an action to activate, that action isn't a function of the Use an Object action, so a feature such as the rogue's Fast Hands can't be used to activate the item.

I totally ruined the formatting of this post and couldn't delete it...

Quote from Devs_and_Dice >>

Are you sure about interacting with an object for using the wand?

Found this:

If an item requires an action to activate, that action isn't a function of the Use an Object action, so a feature such as the rogue's Fast Hands can't be used to activate the item.

 For anyone's reference, the above quote is in DMG Ch. 7. You are right. Specific magic items might tell you what you have to do to use it, and that might be an action, but it is not the Use an Object action as I stated. 

This shows up in equipment on the character sheet, but it needs the option to "Display as an Attack" like many spells have.

I forget sometimes it is sitting in my equipment section when I am in combat as most of my attention is focused on the Actions tab and having it be right there would really be nice.

An option like this for any magic item with an attacking element would be nice.

If you turn this into an Arcane firearm using the Artillerist feature.

Would that add 1D8 per dart? Or only on the total of dmgs dealt?

I believe  that the artilirist specifies ' roll a d8. you gain a bonus to one of the spell's damage rolls equal to.."  so I think that they're all individual rolls so only one extra d8. same for scorching ray.

I find myself wondering, if there is any way to make a magic missle wand as an infusion for the Artificer? That would be a solid choic

It needs to be an artificer spell to get the 1d8 from the firearm so Magic Missile wouldn't proc that effect

With XGtE, you can craft it w/ 2 work weeks and 200 GP.

Thanks. I had an immovable rod that was for sale and I put 50 GP, but next time I run that adventure, I'll put 200 GP.

oh yeah? how does that work?

The Evoker's Glock

The Basic Rules and the Starter Set rules both state that you can use 1-3 charges per use; the DMG and some online resources I've found seem to suggest that you can use all 7 charges at once - for a total of 9d4+9 damage. The Basic Rules are newer than the DMG - should I take this as a correction to the DMG? Or does the DMG overrule the other sources? My 4th level player being able to cast 9 magic missiles seems overpowered.

Even though the description above lists the Basic Rules as the source, here's what the Basic Rules say:

sick bruhhhhhhhh