For wands and scrolls (i.e. the Swiss Army Knives of Wizards and Clerics)? I never found that to be so. Know the spell? Having the money and time? Bingo, there's your magic item.If you follow the rules for making magic items in the DMG, it is pretty hard for players to just make what they want to order.
Again, this isn't what I found at all. You haven't really made the challenge tougher for the casters, perhaps more frustrating, but not harder. What you've done is made the encounter significantly harder for the melee guys.Off the top of my head, Spell Resistance? Something that a wizard's spells don't effect as well will be less of a challenge for fighters and more of a challenge for wizards.
If the casters can't affect the creature, that just puts more pressure on the melee characters to get up close and whittle away its HP. Without spell support from the party's casters in doing that, the opponent lasts longer, devotes more attention to killing the PCs who are actually hurting it (i.e. the melee guys) and usually (IME) ends up killing someone who is a non-caster. You'll rarely see a caster die in this situation because they would be either 1) so inefficient at hitting stuff with melee weapons that there is no good reason for them to get close to the creature (read: Wizard) or are out of melee casting support spells like buffs and healing (read: Cleric). You might very well see the Cleric using up more spells during this encounter, but that's because it's going to take all of his healing to keep the melee guys going long enough to win. After that, what does the party do....? Do they press on knowing that the melee guys are going to get hit, need healing and it's not going to be there or do they rest? IME, the party rests, which means the casters get their spells back.