Hello there, I'm not such an "experienced assassin" but i can tell you some triks and perps teach you something.
1) It's a matter of tastes, i personaly recomend Blade Assassin, why? Because I'm one? No. the reason is because it can skill while attacking with your weapon, daggers or blade, { if u use a bow without poly to do pvm u're insane, but u can do it}.
In pvp? Well it's hard to say since i've never done an Arcer: in my opinion blade overall is best, very hard to make it good in pvp since u have to use tactics not just press buttons and run, u have to wait the right timing for using your skills (this is the main reason i like it so much)
2) I hardly recomend to farm with a party, but if u can't find any... well try harder!
Just joking.
Stats Build is conected to your playing stile: since i'm talking about my personal experience:
- personal suggest put all stats in STR to max, i don't use horse so attacking from ground is usefull, after put 2 dex and 1 vit. Why so strange stats? whell they are not strange at all, you can get plenty of Hp from Hp bonus on items, Defence From an armour +9 or +6 ( the diference is only 18 defence... 
) and a stone of defence gives you more than enought defence for pvm [if u wanna over do it put a defence +6 and you'll become the solitary player frustated and quit ahahahah Joking again.]
3) Best bonus: ahhh here comes the nice part :Q_
Well then if you're still reading me... i hope no...
-I strongly recomend, on every item that can have it 2k Hp or in alternative at least 1.5k Hp;
-Another perfect Bonus for pvm, also for pvp, is 10 Critic on Neck, Weapon and shoes.
-High Awg on weapon, like daggers with 45+ awg will be fine,
-Or a bow with 40+ awg it's ok, just remember to use the bow u need a poly marble and they don't greew up in trees.
-Armour bonus will be like 2k Hp, 10 Hp absorb, and stuff ...
That was just an intro to "general bonus".
Now i'm searcing for this bonus:
_ WEAPON: 45 + awg daggers+9 ss +6 / 40+ bow+7/+8/+9 ss +6;
<Rarity: 8% awg, 5% mob,
<Normal: high awg obviusly, 10% critic, 20% vs (mob u intend to kill), 12 str.
_Armour
<(Mistic or Legendary: Good Bonus are Hp Restore, % Block, Awg Resistance __ I prefer something Like 8% Hp Restore + 5% block)
<Normal Bonus: 2k HP, 10Hp absorb, 50 atack.
_Helm:
<Rarity - they are seling assasins helm with 5% mob at an abordable price lvl 60 helm with 20kk is fine.
<Normal - 8% attack speed, 15% arrow, 20 % vs (mob u intend to kill), 8% poison,
_Shoes: Phoenix ofc.
<Rarity: - 8% awg + 5% stun, perfect combo, well i have 5% awg with 3% stun, if i remember well....
<Normal: - 2k Hp, 10 Critic, 8% stun, 8% attack speed, 15%arrow,
_Neck: -SCN for mobs, -HTN for boss,
-SCN:
<no rarity possible.
<Normal: 2kHP, 10 Critic, 8%stun, 15% arrow,
-HTN:
<Rarity: 800hp+10%resistance (everi mob have a specific tipe of atack and if u have some resistance like: Wind, Lightning, earth, etc...)
<Normal: 2kHP, 10 Critic, 8%stun, 15% arrow,
Bracer: HTB or Ebony (i suggest HTB for more ctitics):
<Rarity: 8Hp Restore, 5% stun,
<Normal: 2kHp, 10% Hp Absorb, 20% vs (mob u intend to kill), 10% pierce, 20% drop.
_Earings: SCE ofc for pvm,
<rarity, No rarity is possible,
<Normal: 20% vs (mob u intend to kill), 20% mov speed [never a bad thing], 20% Drop,
_Shield: Lv 61 or 81
-LV 61
<Rarity: 8% awg, 5% block
<Normal: 20% vs (mob u intend to kill), anti stun (if the mobs have stun % ), 12 str, 15% block,
-LV81:
<No Rarity;
<Normal: 20% vs [b](mob u intend to kill), anti stun (if the mobs have stun % ), 12 str, 15% block,
That was a list of almost perfect bonus, perfect bonus that i consider now with my personal knowleadge.
If any of you disagree with me pls tell me maybe i can make a more eficient equip.
Overall i suggest you this 5 bonus: 2k HP, Hp restore & absorb, critic, stun, block.
4)Skill Build: Well for arcer i dunno... ???
-Blade i suggest you to make it this way:
1st Ghost Attack,
2nd Spinning Dagger,
3rd Fatal Strike,
4th Poisonous Cloud,
5th Teleport Strike,
6th Stealth,
5)Well ansswer your self playing, some things that i like you may hate and reverse.
I like playing with an assassin it's hard, very hard! Damn f***ing hard! But is the class that boreless over all.
One reason is you're more eficient from ground like some ppl with mount and pet and stuff from item shop,
Another reason is best killer in al the game. maybe not the best to tank or the best to hunt metins... but if u're ding a run the sins is the best, just because you don't have self buff so you're always in disavantage .... and when other classes are in disavantage them don't have a good reaction, while assassins are always in disavantage... and we don't complin it soo much.
At the moment i'm farming with my assassin lvl 75 (that's my main acount) and i enjoy it, making much more money that i could have made with farmers at low level, and i'm not talking about events, i mostly farm out of events... it's more funny.
What am I farming? everithing... metins, gliphs, ice marbles, dt.
Try killing a metin in Poly (the best and funny experience ever) like 25 second for a metin that in average takes 1minutes and 45 seconds.
Last of it: if you are still reading you are crazy pls don't, ahahahahah.
Just enjoy the game like it is.... and remember to enjoy more the game you have to be hight lvl as assassin, you can't stop and shit around at lvl 35 or 55, barely at 75 to make some FAST items and go up! ahahah
Chers 
P.S.: I purposely skiped pvp, when you will have good pvm gears you will have also the knowleadge to make good pvp stuff.