What oil and filter do you run, and for how long? You shouldn't need an engine flush. Who told you your oil "looked sludgy"?
Below are some things that are usually changed by 100k miles, if not earlier. Granted, most of them, people don't mess with until there are problems.
1. Spark plugs - this may or may not help with your rough idle. Maintenance guide says these should be replaced every 100k miles. Depending on how mechanically inclined you are, you may attempt to try this to save some money. This is probably what will cost you the most if you have it done by someone else. At least this engine isn't the one where plugs break off.
2. Fuel filter - Guide says to replace this every 30k miles, though most people don't replace until they have issues with starting. If it's never been changed, I'd do it since it's cheap (< $20) and it's relatively easy to replace.
3. Engine Coolant - Might not be a bad idea to flush or change out your coolant. This should also be relatively easy to replace. While you're there, check for coolant electrolysis. Stick the positive lead of a multimeter in the coolant, and the negative end touching the negative post of the battery. If it's more than 0.2-0.3V, it's conducting electricity, and is potentially eating away at your radiator. If you replace it, make sure it's a coolant of the same color.
4. The serpentine belt wouldn't be a bad thing to replace. A good one (Goodyear gatorback) is around 30 bucks, though I'm sure you can get cheaper ones. This can be a pain to install the first time (especially if you forgot to take note of how the belt goes
), but if you take your time, it's relatively easy. You can rent the serpentine removal/installation kit from Autozone/Advance Auto for 20 bucks, which they refund as soon as you're done.
When I did mine, I stored my old one on top of my spare, in case my current one ever broke. As JUST4FUN said, if it ever breaks, you'll pretty much be stranded. Happened to my sister, and her car had to get towed. Better to pay 20-30 dollars now, than to pay 100+ to get it towed.
5. Air filter - You didn't mention changing this, though I'm sure you have. Either way, a dirty air filter hurts gas mileage.
6. Drop the spare, and inspect it. If anything, grease up the carrier, and check for rust. Wouldn't want it to not lower, or to have a flat spare when you most need it.
7. I know you have your other post regarding this, but I figured I'd include it here. What color is your automatic transmission fluid? Is it pink, a darker red, or brown? Does it smell burnt?
