For lesser crafts, its definitely the jewel, as Eld runes drop frequently (I'm assuming you would want fresh runes, so you could get ilvl91+ crafts to ensure 4 affixes...the highest alvl on boots is only 65 (of Transportation)), and boots(caster) and light plate boots(blood) are shoppable (though boots are
not in Hell, so you may have to carry a pair in stash, or be prepared to gamble for them (still not hard, just kinda pricey))
For greater crafts, its either the base (caster takes wyrmhide boots, not shoppable and can't drop normally, must be racked or gambled) or the Sol rune (blood takes mirrored boots, which can be shopped (hell Jemala or Halbu, and Larzuk)
However, bases, jewels and gems (remember that any quality of gem works, they needn't be perfect) can be carried in stash...but if you want the highest possible ilvl on your crafts, you have to craft with a rune in the same game it dropped in, as it will lose its ilvl upon S&E, so in that sense, its always the rune that is hardest to get...
Crafting runes in 1.07: Eld/Sol for lesser/greater boots, Tir/Shae for lesser/greater gloves, Nef/Dol for belts, Hel for shields (no lesser shield recipe), Ith/Io(Po) for body armor, Ko for rings, Lum for amulets, Ort/Fal for weapons...
The ideal environment for crafting is actually an MP game, as when you get a crafting rune drop, you can then join with a character with the necessary base, jewel, and gem without having to tie up all that stash space
Not sure about the possible max on a rare MPK ring...while I've never rolled one with over 37MPK (the max on a magic), or seen someone else's, that doesn't mean its not possible to get more... given that 37 is >31(max value with 5bits), I suspect that up to 63 (max value with 6bits) could be possible, maybe more... and I have no clue if the CR from an @rez affix would be converted, either...
Edit: LOL, I very nearly said something about asking
@Fruit to check on this... but he hasn't been active on the forums lately, so I let that lie...
WoRG