If your engine is really idling at 280 rpm, then it won't be long before it locks up. That's not turning fast enough for the oil pump to get oil to the top of the engine. Cam bearings will lock up. It's a good thing that it seems like you might be reading the gauge wrong from your description. Just barely above the first mark would be more like 600-700 rpm. At 300 rpm, your engine would be shaking and sounding like it was in trouble. You would have a hard check engine light. That first mark on the tach is more like 500 rpm- not zero. Correct warm engine idle in N should be 700-750 rpm.
To verify this, use the electronic odometer trick. Key off, press and hold the trip odometer reset button. Turn the key on, and continue to hold the reset button until the odometer reads [test]. Now release the odo reset button, and you're in test mode. Now each time you press and release the reset button you will scroll through one of about 30 test functions. I think the tachometer is the first one you come across that reads [ 0] on the odometer. Be careful though because there is another one that reads the camshaft rpm which is 1/2 crank rpm.
As far as a "New To You" used car, you've done about all you can do on that particular model. For a CAI, I recommend the "Stealth CAI" trick first to see if you like it. You will want to add 100% polyester panty hose over whatever generic 3.5" cone filter you use. This, if installed loosely over the filter, will not reduce air flow but repels water. It's an old dirt bike trick to keep mud off air intakes. It is important not to submerge the front of the car with the engine running- all the panty hose trick will do is prevent spray and splashed water from being sucked up.
Some things you might want to check before you go that route (coffee is kicking in)- the fuel filter. Often overlooked and never serviced properly, it's not that hard to replace. Start a new thread if you need advice- if you think you have to remove the plastic clips from the lines in order to disconnect the fuel lines- then you need some help. Quite simply- press the the white rectangle in, wiggle, and pull to remove. There is one you can't reach on the fuel filter, follow that line away from the fuel filter, and you'll see another clip that you can easily reach after about 6". Disconnect it there, and remove that small section of line with the filter. There are more directions, so if you need advice, like on how to discharge the fuel system, please ask.
The coil boots. Mine went out about 80k miles. Yours might have been replaced, and might not. The boots are easy to replace, but might seem more expensive than regular plug wires. Boots cost $5-8 each, and come with a new spring and solid conductor. I also had some corrosion on the coil-spring connector. I used electrical parts grease to prevent future corrosion. We have a few online retailers that we get discounts with- like Village Ford
or Rock Auto. Please compare their prices to your local price.
One thing you don't want to look at right now...... the price of the thermostat for this car. I'm at 150k, and have had no thermostat problems- cross your fingers. I can tell you that I also get about 28-30 during this time of year, and it is not as cold down here as it is up there. I have had luck blocking off part of my radiator like big rigs do in the winter. Using the electronic odometer trick I can monitor engine temps in C, and it doesn't approach overheating at all. The engine does stay warmer, and there is less thermostat cycling once it is warm. 05 Focis have a different thermostat (the cheaper one!) than 06-07, and it might be a 180F instead of 194F. It sure seems to be lower when watching the temp on the EO. On the EO, scroll through until you see a readout that looks like this [ 15 C]- the value there is just a guess at what a cold engine might read- use your own judgement about what yours might read. I have had mine half blocked off up until the outside temp was 60F with no issues. If you try to do it- that front cover over the radiator slides towards the front of the car- then up to remove once the plastic screws are removed. If you use cardboard- cover it completely with duct tape to reduce water intrusion. I use retired big rig mud flaps trucking companies have to PM those more often than you think.
Good luck, and let us know if we can help more.