Well your first purchase/grind is going to be for Willpower Jewelry. There really isnt a better substitute at the moment. If you can get all arcane, it is certainly in your best interest. If for PvP, its not the end of the world to have a healthy or stam piece. For PvE, its a DPS loss. Arcane rings are about 100k, so they arent cheap, but they are worth the gold. If you are going to skimp here, at least buy an arcane neck. You can get the other rings much cheaper then replace as you get more gold.

Seducer/magnus is actually a great set for leveling. Its also not a terrible set for PvP because of all the sustain. PvP frankly gives you more options because there is more than one way to accomplish the same goal.

For PvE: The current meta is to stack Damage and go Destro/DW. The standard for a DPS Siphoblade is probably: 5 Julianos (6 trait), 3 willpower jewelry (IC vaults or purchase), a 2 piece monster set like kena or nerien'eth, 1-2 torugs pact weapons (malestrom staff if you got it, otherwise you want nirnhoned). Monster sets will be available for purchase in the next DLC it seems. Might make sense to wait for next patch to craft a bunch of gear.

For PvP: You need to find a balance of damage, defense, and sustain. Personally, I would probably go Destro/Resto for PvP over DW/Resto. I like the range of destro staff and the ability to weave attacks. But if you want to get up close and personal, you can definitely make DW/resto work.

From a crafted set standpoint, I would probably go 5 Julianos, 3-4 Magnus, and 3 willpower. If you bring monster helms into play, then the 3-4 piece magnus would become a monster set and 1-2 piece torugs pact. Armor should be divines (1-2 pieces of impen is not bad for PVP, but useless in PvE) and weapons should be nirnhoned (resto staff can be precise).

EDIT: Something to strongly consider. If you plan to run Monster sets down the road, you might want to make your swords in Torugs pact and skip magnus all together. Getting gold v16 weapons is the most important thing from a crafting standpoint. Nirncrux is not cheap. A gold v16 sword in nirn is probably the most expensive thing you can craft. It would suck to make them in magnus, get some monster pieces, and have to do it again. The 3rd and 4th bonus from Torugs arent bad for PvP, so you could just make you head and shoulders in Torugs to start with as well, perhaps in v15 so they are easy to replace.

When it comes to crafting, I suggest making your main 5-piece set (julianos) from your chest, legs, belt, boots, and gloves and your other set from head, shoulders, and weapons. That way you can easily swap in monster sets if you want to down the road. The other thing to consider is going 5 light, 1 medium, 1 heavy, but that really only matters if you plan to level undaunted for the last passive. It takes a lot of dungeons, but is definitely worth it. If going that route, I would make the chest heavy and either head or pants in medium (doesnt matter which). This will give you the best armor rating.

For enchants, enchant ALL your armor with magic. Use your attributes to hit your desired health. Much cheaper to make adjustments that way. For weapons, your sword enchants are meaningless because you shouldn't be weaving with them. For Destro staff, it depends on how you stack CP. If into thaurmaturge, then you want poison, if into elemental you want fire. Disease is also a solid option for PvP becuase of the healing Debuff. For resto staff, I personally like the magic return enchant, but that is really more from a PvE standpoint. Dont overthink the weapon enchants too much, they arent that game changing.

How do you want to play?

Up front in melee range all the time or ranged at times? It will determine what skills would be good to use.