Depending on how many miles your car (condition of stock shocks) has done I'd save the money and just go for Tein springs. They say that they only lower like 30/20mm but in reality its a lot lot more. Have a search on here for lowering, I think it gives the car a good stance, and considering their a fifth of the price of coils, cant argue.
However its your money, depends how long you intend on keeping the car.
As for the other questions
1. See above (not really in my opinion)
2. Yes but you'll probably need somewhere to redo the suspension geometry
3. Trial and error
4. Pay the extr for HSD's in my opinion, meant to be the best money can buy.