Dont have the dealerships do this work. A reputable mechanic shop can install springs for 2 hours of $79/ hr shop rate. Pluse $70 for a good alignment after. Lowering springs void a warranty? (per Fireblades comment). Never heard of that, must be a California thing.
I think the biggest factor here (aside from budget) is the amount you are looking to lower the car. For drops up to 1.5" its my opinion that a quality set of springs are sufficient. If you are looking to lower the car beyond that into the 2"+ neighborhood, then coilovers are the choice.
A "ton" of miles and becoming "boring" to drive. First ask yourself, do you want to continue in a Corolla, or have you outgrown it? If you are looking to make your car "fun" to drive also think about what it is you like. Are you sure its suspension upgrades you need or do you want to give...
The price seams right there. I looked up the year/make/model on KBB and rated it at "Very Good Condition"...price for private sale was $6245. I would say mechanically you are sound, you should double those miles easily enough. Expect only to possibly need to replace the wear items like brake...
Rear UR strut brace went on tonight...you will want a deep well socket, 14mm, for best result. At least I did. First drive will be tomorrow and I will determine then if I want to do any further bracing, i.e sway bar, H brace...
If your OE is a 205/55/16. You can move up 2 sizes without issue
Or if you had the 215/45/17 you can move up the same 2 sizes..
Either way your 18 will fit diameter wise...What you need to be aware of also is offset. The position of the mounting serfave of the wheel to centerline.
I was just thinking...what if you put the wires inside some red (or any color) conduit? I've seen it in the auto retail stores where it has the split and simply wraps around any wire...granted its still running over the top, but would look much better...just a thought.
I really am digging that red powdercoated HPS intake (or any of equal or better quality) but most reviews Ive read they are throwing CEL. Im not wanting that so Im putting off...:-(
Ive heard talk of the Weapon brand header for the Corollas...you can start here. Just smart search into any search engine to find a retailer. There are literally dozens out there. Weapon-R.com Carid.com Tunersports.com these just to name a few. :-)
with all the above info, I think the only thing I can add is tire pressure. Keep those tires inflated properly and check them regularly and adjust as needed. An underinflated tire(s) will eat into your MPG.
Headers are just a more free flowing exhaust manifold. This then leading to the resonators and the muffler. The more air movement you can get into and out of any motor, the more power it will make. Based on its limits obviously. :-)
I've always had my suspensions about any fluid, be it oil or any other, when auto makers put their name on it and call it theirs. Although there may be additives to a specific fluid an auto builder recommends, Toyota does not own any oil refineries. Lol. So "Toyota fluid" of any type can be...