You can apply class abilities to spell casting from magical items
While casting from an item is not the same as a Casting a Spell action, it is still casting a spell. Unless a term is given a special definition in 5e game terms, you use the common definition for that term. You are casting the spell, the ability to do so comes from the item. For instance, the Staff of Fireballs:
.. you can use an action to ... cast one of the following Spells ...
Is no different, in this regard, to Wand of Fireballs from:
you can use an action to ... cast the Fireball spell ...
Jeremy Crawford has posted a number of answers we can point to in the time since this question appeared that verifies this really is the case. Each time he's been asked about a specific class ability the answer has been an affirmative. These were made when his tweets were official rulings, but since have been "downgraded" by the Sage Advice Compendium to his personal advice and perspective. While this is true, it still is clarification of the intent of the designers.
Metamagic
Can Metamagic be used on magic item casting a spell? Yes.
If a magic item's description says you cast a spell from it, you can use Metamagic on the spell. #DnD
and
Metamagic works w/ any spells that sorcerers cast. Wild Magic Surge can work w/ any sorcerer spell they cast. #DnD
Wild Magic
Can using a magic item cause a Wild Magic Surge? Yes
Yes.
Arcane Ward
Can casting from a wand/staff recharge Arcane Ward? Yes
Arcane Ward/Twinned Spell works when you cast a qualifying spell. It even works when an item says you cast one. #DnD
The ring of mystic fire only affects the upcoming spell once
The ring of mystic fire (Magic Item Compendium 125) (7,500 gp; 0 lbs.) does, indeed, grant "a bonus to the amount of damage you deal with the next fire spell you cast before the end of your turn," but the ring only grants that bonus once. Each ray of a scorching ray spell isn't the spell; instead, the whole spell creates multiple rays. The ring improves the spell's damage only once by a maximum of +4d6 no matter how many effects result from the ring-modified spell.
(Ruling that each ray of the scorching ray spell benefits from the ring multiplies the power of the ring by the number of rays—with rather extreme consequences at low levels, especially given that the ring increases also the wearer's caster level for fire spells!)
Although the DM has several options available to adjudicate the ring's effect—allowing the player to distribute the total number of extra dice damage the ring grants among the rays before (or even after) the rays're launched, for instance—, this DM would keep things simple and have the player add the extra damage to his PC's first ray in a volley from the spell scorching ray, in much the same way that precision damage applies to the first attack in a similar volley (Rules Compendium 42). (In fact, I have a fire wizard in a current campaign who's eyeing this very item, and that's how I'm ruling it works.)
Hence, for example, under such a ruling, a level 6 caster wearing a ring of mystic fire can take a swift action to spend 3 of the ring's charges. Then the caster can cast scorching ray, creating 2 rays (because of the increase in caster level granted by the ring) and making 2 ranged touch attacks. If the first ray hits, it deals 8d6 points of fire damage. If the second ray hits, it deals 4d6 points of fire damage.
Best Answer
Some examples of magic items that accomplish similar things:
Spell Scrolls: each spell level has a given rarity, save DC, and attack bonus. The attack bonuses progress from cantrips to 9th-level as +5, +5, +5, +7, +7, +9, +9, +10, +10, +11.
Circlet of Blasting: Casts Scorching Ray with an attack bonus of +5.
Ioun Stone of Reserve: Casts stored spells with the attack bonus of the caster who stored it.
Quaal's Feather Token (Whip): Makes spell attacks with an attack bonus of +9.
Ring of Spell Storing: casts using the attack bonus of the person who stored the spell.
Ring of Spell Turning: When it reverses a spell, it uses the attack bonus of the original caster.
Staff of Power: When casting its spells, the user's save DC or attack bonus are used. Note that this item is restricted to use by certain spellcasting classes. This also occurs with...
Wand of Paralysis: Makes a spell attack using the user's bonus. Also only usable by a spellcaster.
So the first thing you need to consider is whether your item is restricted to spellcasters or not. If it is, you will probably be using the spell attack bonus of the user. If it is not restricted, the trend seems to be to assign a specific attack bonus to it, which seems to increase with the rarity of the item. If it is not restricted, the second step is then to decide what rarity you would like the item to be, which is best cross-referenced with the spell scroll table to find an appropriate spell attack bonus or DC.