To tilt the frame slightly, instead of increasing your caster levels to ridiculous heights, you can try to avoid getting your spells dispelled by just becoming immune to spells that dispel magic. While many spells dispel magic, unless the DM is Dumpster-diving to deliberately overcome your shored-up defenses, you only really need worry about the 3rd-level Sor/Wiz spell dispel magic [abjur] (PH 223) and the 6th-level Sor/Wiz spell greater dispel magic [abjur] (PH 223).1 The basic items that help do this:
- The ring of counterspells (DMG 230) (4,000 gp; 0 lbs.) counters once the spell that's cast in it after which it can be reloaded with the same or a different spell.
- The ring of spell-battle (Magic Item Compendium 127) (12,000 gp; 0 lbs.), in addition to other effects, grants the wearer 1/day the ability to redirect a spell that the wearer's identified that's cast on him at a different target within a limited range.
- The magic weapon special ability spellblade (Player's Guide to Faerûn 120) (6,000 gp; 0 lbs.) is keyed when created to one specific spell (like dispel magic or greater dispel magic but not both), and when that spell is cast upon the weapon's wielder he can either redirect the spell to the originator or have the spell dissipate.2
Note that of these only the ring of spell battle can, after its fashion, defend against area dispel effects.
Other similar magic items likely exist—as do shenanigans (like finding someone with the feat Craft Contingent Spell (Complete Arcane 77) and paying him to ensorcell you appropriately)—, but the above list probably holds the most convenient and palatable solutions.
1 Other spells that dispel magic include (but by no means are limited to) the 2nd-level Sor/Wiz spell arcane turmoil [abjur] (CM 96),
the 2nd-level Sor/Wiz spell dispelling touch [abjur] (Player's Handbook II 110), the 8th-level Sor/Wiz spell blackstaff [trans] (Magic of Faerûn 81), the 8th-level Sor/Wiz spell chain dispel [abjur] (Player's Handbook II 106-7), and the 9th-level Sor/Wiz spell reaving dispel [abjur] (Spell Compendium 169-70).
2 Many weapons can be wielded—often simultaneously—an not occupy a creature's hands. I suggest multiple surprise weapons (Complete Scoundrel 109-10).
Preamble
Let me start by stating two points that are relevant to this answer. Feel free to skip this section if you want to jump directly to the 5e material.
In previous editions, official FR material dictated that paladins receive their powers from deities. In general, deities are a major part of this polytheistic setting, and mortals and powers are well-aware of this. Deities get to run the (meta)physics of the world and not having faith diminishes their power and causes imbalance. Our real-world secular/religious sensitivities are not attuned to this way of thinking, perhaps a good analogy is having your children vaccinated. When most people have vaccination, we get a herd immunity; likewise when most people have faith, the FR world 'runs'. As such not just paladins, but all people are expected to have a patron deity. If not, they are "disciplined" in the afterlife and become part of the "Wall of the Faithless". The attempts by the god of the dead, Kelemvor, to resolve this seemingly unfair treatment of the faithless has been depicted to have serious consequences in novels, which resulted in a change of his personality.
In spite of the central role faith plays in the FR of Greenwood, you are more than welcome to reject it; or find ways around it (like a faithless paladin who is still being watched over by a deity like Torm). Throughout its whole history, Ed Greenwood and the various versions of any written material have always highlighted the fact that each FR campaign belongs to the individual DMs. So feel free to rule however you like.
As for the officially printed material you have asked for, below is what is available.
What do the 5e Sources Say?
Sword Coast Adventurer's Guide states (page 131):
When ... a warrior also has great devotion to a particular deity, that god can reward the faithful with a measure of divine power, making that person
a paladin.
And yet on page 132:
Most paladins in the Forgotten Realms, like clerics, are devoted to a particular deity.
These two sentences might seem to be in conflict, unless we interpret them to state that 5e FR paladins get support from one or more deities. This interpretation is supported by lore from earlier editions (see for example the Triadic Knights) as well as the text on clerics (page 125):
A typical cleric in Faerun serves a single divine patron, but some individuals feel called to serve a group, ...
D&D Adventurer's League
It is worth noting that the D&D Adventurer's League FAQ has switched its position on this issue over time. (The change might be correlated with the introduction of non-FR campaigns to DDAL.) In version 2.5 it stated:
Does My Paladin Have to Worship a Deity? Yes, though your character’s alignment isn’t required to match that of their deity.
While version 3 states:
No. What makes you think you do? Dunno what you’re talking about.
WotC Suggestions for Content Creators
Finally, Wizards of the Coast has a set of style guides for their official play writers and have been made available for all content creators on DM's Guild. Note that these guides are suggestions and not rules, but for the sake of completeness, I would like to quote the relevant parts. D&D IP Guide states:
Divine magic is practiced by the most devoted followers of D&D’s gods: clerics and paladins. ... Paladins are the holy warriors of D&D. Like clerics, they serve a deity and channel divine power to achieve their goals (or the goals of the temple or holy order they serve).
Forgotten Realms Style Guide confirms:
Paladins are warriors of unusual virtue and piety rewarded by a good god with divine power. ... paladins are typically judged more swiftly and harshly by their gods.
Best Answer
The DMG on page 287 "Changing Proficiencies" specifically explains how to do this. Trade your armor and weapon proficiencies (except quarterstaff proficiency) in exchange for something along the lines of the monk's Unarmored Defense feature.
There's also one other minor change you should make. Your character wants to be a caster, but many of the cleric domains assume you are a melee combatant, and give the player the Divine Strike feature, which improves their melee damage. If the player chooses a cleric domain with the Divine Strike feature, I would also let them trade that for the Potent Spellcasting feature from the Light or Knowledge domain.
It's also worth noting that the Monk's Unarmored Defense feature is significantly less powerful than medium or heavy armor + a shield. The DMG presents this alternative as a guideline, not a strict rule, so you may want to consider giving them an extra +1 or +2 AC on top of the unarmored defense, or if you think that's too much for a backline caster, throw in some other miscellaneous perk instead.