You may want to try this build out before it gets 'fixed'. Or at least, Cast on Crit, in general.
http://www.pathofexile.com/forum/view-thread/706896
As for your questions, you can get life leech from gems and as mods on accessories, weapons and uniques. There's also a few passives for it (one for life leech is by Marauder, one for Mana Leech is by Shadow). You can get life on-hit from mods on weapons and uniques and passives (two near Marauder and I think there's one Claw node with it) and support gems - life on-hit doesn't work with spells, life leech on the support gem will work with spells, but there is no mod for spell life/mana leech yet (there maybe a unique for it, but I don't think so). If you want to get the most life leech, pick Shadow, grab the claw passives, get some claws with +life leech spawned ontop of the innate life leech, get a couple uniques with +life leech, then run Blood Rage and a life leech support gem on your attack skill. A guy in my clan has 28% currently, he doesn't even need life flasks.
With regards to crit, you have base crit, usually 5%. If you get +100% increased crit from the passive tree, you have 10% crit chance. If you use Critical Weakness curse, it's now 15% I think (it gives a flat 5% at rank 1, though I don't think it applies to your base crit chance). If you get something with 6% base crit and apply 100% increased crit chance, it becomes 12% base crit. Going to +200% increased crit will only make your crit 18%, and so on.
Critical hits are tied to the accuracy mechanic, because evasion "secretly" reduces enemy crit chance. If you would otherwise miss, your crit will let you score a normal hit, if you would otherwise crit, evasion may lower your crit to a normal hit. I don't know how this works exactly, but it does. Your opponent can never have 100% evasion, a level 1 character has a 5% chance to always hit somebody. Only thoguh keystones can you give up your innate 5% chance to evade. Crit is thus capped at 95%, but may be effectively 100% against people with "cannot evade" - but I don't know this for a fact. Minimum crit chance is 5% - and this is what's tricky - if you have something with below 5% base crit, it
has 5% base crit, but when you increase the crit chance, it will use 4% or 3% (that base chance, I don't think there's a skill below 3% base other than ones that have 0% crit chance and cannot crit on their own, like unarmed attacks) increments instead of 5%, so the base there only applies to increases. It's not a good system for new players to understand, that's for sure.
It's also worth mentioning that evasion in PoE is
deterministic. It's not random, if you have 50% evade chance, the game tracks hits by your opponents - a single opponent will miss 5 out of 10 hits, always. The game tries to prevent too much streakyness (unless you really do have a high evasion chance), but there is a little randomness in when the hits and misses will occur. Crits, however, are totally random, or at least, that's how they were the last I read a post from Qarl about it. This is why Ondar's Guile is totally broken and why evasion is really strong, you can achieve 95% evasion chance with 50% base evasion chance vs projectile attacks (And using grace, it's rather easy to get 50% evasion chance vs enemies your level) - this effectively makes you almost immune to ranged crits.