On Pottermore we can read, 3 paramaters define the wand: the wood, the core and the lenght.
The wood used for a wand come for tree know for have some magical properties (not only in Potterverse, but in our real world), this parameters define the type of magic you can perform easily with one wand (transfiguration, healing, duel...)
All the wood used for wand by Ollivander came for old trees garded by Botruc (i don't have any idea of the English name of this creature. You know, the little creature, look like made of wood, they studie it during lessons with Hagrid)
For the core, Ollivander's used the three great core, licorn, dragon and phenix.
Licorn made wands not designed for dark magic with some complex specifities.
Dragon wand is more destructive, but the principal advantage is the rapid learning and facilities to use new spells.
Phenix is the most powerful and everyone wants à Phenix wand, but it's really powerful if you are compatible (or if you have conquered it)
Other core exist (Thestral, Velan hair, Kelpie hair....), but we don't need to talk of it.
Basically, a wand lenght is somewhere between 6 and 12 inch. A long wand suit a powerful sorcerer (Big V, 14"8' i think, i know the lenght in centimeter but not in inch...), or a very tall one (Hagrid, 16 inch)
A short wand suit a small sorcerer (Flitwick) or a person with mental and personalities problem (Umbridge, 3 or 4 inch, can't remember. I'm serious, you can find all of it in Pottermore)
Why not metal wand, or magical renforced wand?
You can decorate your wand with some metal things and carry it in a reinforced device (Lucius cane is the best exemple i think, but if you look the pommel is fixed on the wand for a better grip probably, and he use the cane like a sheath, for protect it) but you can't made a wand of pure iron, reinforced steel or an other metal or hyper resistant material because they don't have magical properties.
If you can find something with magical properties, like magical wood used, and with a better resistance you probably can make a wand with it (maybe a magical creature bone? Like the Basilisk maybe... )
And I don't think you or an other wizard can just reinforced(enchanted) your wand with an other wand, your wand probably rejected the magic (1)
Maybe a potion or a sister-core wand can? Or if you are good enough with wandless magic, you can put your own raw magic to your wand for protect it.
The Elder Wand can do thing look like impossible: repared a broken wand(2). Maybe if you mastered it, you can enchanted your wand without cause any malfunction, but if you have the Elder Wand, you don't need an other wand.
maybe it's why Harry's wand can't stay repair when he tries to fix it with Hermione wand, magic rejection and auto-spell breaking but the Elder wand is powerful enough for bypass the magic defense of the wand and restore it.
You can find the stuff about wand and more details about core and wood in Pottermore if you want.
Sorry for imperfect English.