1) The starters on these cars are actually very durable. Problem is that they get dry inside and start to stick. It is very common for an SC starter to not want to engage after sitting for a little while, especially in humid climates. Many times simply cycling the key 20-50-100 times will get it to spin.
The cheap cure for this problem is to remove the starter, dissasemble it, clean it, lube it (grease in the planetary gears, oil on the bushings), toss in some new brushes for good measure (if needed, sorry don't have the part number off hand) and it will run better than a new one. I've seen many reman starters that were put together with such little grease that they stop working in short order as well. It's quite rare for the starter to actually go "bad" to where it needs replacing.
2) As for the rough running condition, make sure that the balancer is in fact an SC balancer. It must not have any counterweights on it (I'm not talking about pins here, I'm talking about an obvious counterweight - compare it to your old one). What year is your SC anyway? Also make sure that the pulley is on right. It only lines up properly one way as one of the bolt holes is significantly offset.
For the most part I have found that with the crank sensor (assuming 89-93 here) as long as there is a gap such that the reluctor ring does not strike the sensor, then it will function normally. Make sure that the shield ground on the harness is connected (this is that small plug next to the round one and it's the one that always wants to break off....)