Questions to learn

I'm trying to decide on how to spend some money wisely to tune up my car (who as some here may have read, has several problems that need serviced), but besides the necessary service, I would like to replace some parts that may not necessarily be malfunctioning, but that could be worn out due to the high mileage of the car (close to 100,000 miles now). So I got a few questions for the knowledgeable people here:

What are the parts that affect performance the most? I know the spark plugs do, but what about the coils? Would I feel a noticeable change in acceleration if I spend $120 in ACDelco coils? What other parts that are not terribly expensive could I replace in order to give new life to the car?

Regarding spark plugs, recently I replaced the factory originals for Bosch 2 Platinum ones, believing that they would be much better. But I read many posts in this forum from people that seem to have all sorts of problems with them, including surging. Even one guy at the Autozone told me that those plugs are not very good for regular driving, that they are better for racing. Added to that, I believe they could be the cause of my reduced gas mileage, 100 miles less per full tank on average. I'm thinking about buying ACDelco plugs, so my question about this is: should I buy platinum or copper? I know that platinum is supposed to give better performance, but does it use more gas at the same time? Because that's my priority, saving gas.

Speaking of gas, my mechanic told me that regular gas is crap, it's dirty and that I should use Shell V-Power. However, I read on several websites that there is really no gain in using premium gas on cars that are prepared for regular. I do however notice the difference when I use regular my car doesn't respond so well, especially if I'm going on an avenue that is slightly uphill and the overdrive gear is on, it kind of shakes a bit, while if using premium gas it's smoother. As of now I'm using medium quality, but it's hard to figure out who's right, because my mechanic has 20+ years of experience in this, but then I read all these websites that basically say the opposite, so I would like to know what you guys think about it.

But going back to more important issues, I'm trying to determine the cause of my reduced gas efficiency. I read in another forum that a possible cause for that could be the O2 sensor, which I replaced last Saturday, however since then I filled up the tank, and now it's 144 miles on this tank and the gauge is in the middle, which tells me it will probably be on the E at around 250, since it's not uniform, it usually takes the gauge a while to get to the middle, but then it starts going faster towards the E from there. So my question is, what are the parts that can cause such a drastic reduced fuel economy if malfunctioning? Would it necessarily be any of the fuel parts, or could it also be due to problems in the ignition system? (which I have)

Thanks for your replies