Don't go too crazy with MP gears; remember you'd want stats when casting offensive spells (except maybe for Dia)--but as soon as you swap, the MP from gear in slot is gone.
I like to run down MP really low, then Convert just before I need to cast Refresh and/or Haste and/or maintenance Curaga (and Reraise, sometimes), so I can be sure all the extra MP from gears I'm likely to swap out soon would be used.
After dropping a Cure IV on myself, I like to Regen and/or Drain the rest of my HP back slowly, so I tend not to use up a lot of MP to cure myself post-Convert--meaning I don't need huge amount of extra MP from Convert. (It's also a good idea to leave some HP bar open for the PLD in party to fill when they need some extra enmity.)
Obviously, in more stressful situations, I'd convert earlier to be sure I have the MP to handle emergencies, and not wait for the "most MP efficient time".
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One place MP gear really shines for RDM is in Dynamis; it's good to be able to keep tossing out Refresh, Stoneskin, Silence, Bind, Gravity, and Sleep/Sleep II/Sleepga when weakened. x_x; (Unfortuantely, keep casting lot's of Sleep and other spells is a good way to become double weakened.)
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Edit:
Since you're Elvaan (and relatively new), you should build your Convert set around:
- Magna Gauntlets (MP+24)
- Magna M Ledelsens (MP+20)
- Magna M Chausses (MP+12) -> Warlock's Tights
- Magna Jerkin (MP+30)
Get those if you haven't already; they're free, and good for 76 MP right there.
- Electrum Ring (MP+20) x2
- Electrum Hairpin (MP+25) x1
- Holy Phial (MP+9)
Those should be relatively inexpensive, and good for another 74 MP.
Not quite as cheap, but useful for hMP+3 besides MP.
If you have the gil... Pretty nice for Elvaan. Probably out of reach.