I would go with a nm holy freeze merc, that seems to be the best. the weapons that are the best are hard 2 get, i use obediance polearm its an eth cryptic axe and it does about 1.2k dmg and it boosts my mercs attack up 10 fold (9.8k), if yer poor go with insight (ral-tir-tal-sol). The armor u could use would be either fort, stone, beast, coh, enigma, or any of those rune words, fort is what most people use. If yer poor use skullders or some other +skills or +res armor you can find. The helm is hard 2 get if u want a good 1, derilium is good 4 em, boosts up there res and skills, u can also go with dream, andys mask, coa, or rockstopper. If yer really poor go with tals mask (good ll)
Well i hope that helps a little.
Depends on where you mf. If you play bosses, defiance should work better. If cows/something like that, holy freeze gives you better control.
Nice equip is as usual.
Make sure the merc has both damage and lifesteal. That fancy runeword (forgot its name) that gives conviction aura is always good for increasing your killing speed. For helms, andys visage or crown of thieves, or even gaze are good picks.
Armors are for example fortitude (expensive), coh (also expensive), stone (decent price), shaftstop (cheap)
The armor u could use would be either fort, stone, beast, coh, enigma, or any of those rune words
Just to chome in with similar advice, when I started MF'ing I used an A2 Nightmare defensive merc (Holy Freeze) with Reapers + Up'ed Shaftstop (Um'ed) + Vamp Gaze (Um'ed). This give max resists in hell, good damage reduction and life leech. It's not that expensive (most expensive are the 2 Um runes). This serves you well and is very cheap (compared to the other options). I like the Reapers because it slows targets even more for crowd control and breaks most physical immunes. So if someone happens to spawn as immune to your damage type and to physical, reaper's should make it so your merc can still kill them.
As an expensive setup which I've been using lately, go with either Infinity (break some immunes for you) or Doom (Holy Freeze aura) on an A2 defiance merc. This boosts your defense since you do most of the killing anyway. I also put a Fort + Andy's mask (Ral'ed). This also give max resists and some above 75 depending on the gear. I've been lucky and stumbled on a couple of Eth Sacred armours with 850+ def before even becoming a runeword. It takes a bunch of Str to wear them but the +Str from Andy's gives it to you at level 83.
Hope that helps with a place to start out and something to aspire to.