I'm in agreement with @InThrustWeTrust on this. You're not really getting gains from the changes you are making. Depending on your rims/tires setup, you may be slowing yourself down. Catback exhausts are just for sound and don't really add power unless you also swap out your headers which, on a Lexus and IMO, wouldn't make any sense. I've never done it, I know that.

Mileage really is only an issue for actual engine mods like forced induction, cams, etc. because you are adding strain to the engine internals which are worn. I would, however, be really careful with what you choose to do with your suspension. Just adding coilovers will throw off your camber, especially with aftermarket wheels. You'll need to buy adjustable camber plates for the front strut towers, install them, and REALLY (I mean REALLY) know what you are doing to dial in the geometry correct and not ruin your tires. I recommend a professional install on that. Also note that changing the suspension adds more stress on virtually everything (ball joints, tie rods, etc.) because all those other parts were not designed to work at a different right height. Parts will wear out prematurely, that's just the nature of the beast. The only way around it is to do a FULL suspension setup, which on my '98 Camaro was over $8k. On a Lexus, I shudder to think.

The intake is also an issue. If you want to gain any power, you have to use one with a larger MAF radius than stock. Otherwise you just have a nice looking engine piece but no power as you are still sucking in the same amount of air. Moving to a larger MAF requires a tune, and I again recommend a professional for this as you will get maximum gains and also be sure you are running Stoich and not lean or rich. Problem is, between the intake and a tune you're looking at $800ish. That's a lot for what will amount to MAYBE 8 - 10 horsepower. Maybe. And of course, all bets are off on longevity as the car is no longer running as Lexus intended.

I would not tinker with it. You have a good car to drive around in, but it is not a good car to modify. Anything, and I mean just about anything, can be made to be fast if you have the money. But I would not invest mod money into an ES350. I don't know if you are looking to road course it or drag race it, but that car will never be good at either. It wasn't made to do those things. And it can be made to, but trust me when I say it would be cost prohibitive to turn it into a competitive car for either event. 

My vote is drive it and enjoy it. Its a really nice car and I doubt you want to mess it up or you wouldn't have asked before doing more changes. Leave it as is and it can really last you a long time. That's my opinion.

Seems like you want to rice it, not really modify it. 

You can kiss the legendary Lexus reliability & luxury goodbye, though.

@mod_man will go into the nitty gritty of your plan of action.

Me personally, I think you’re wasting money for not much gains.

Any car can and will last forever with enough maintenance and bs. Also it's lexus--->toyota luxury brand. You should be fine unless you don't maintain your car at all. Are you having buyers remorse? Cause an exhaust and rims (good ones) would set you back wayyyy farther than a cold air intake and coilovers. They be cheap in comparison.