p0300 misfires can be a little tricky to pinpoint as its a random misfire the computer is noticing.
this is where a bit of trial and error and process of elimination when you are lacking a few peices of equipment. (which is fine, as you cant own every peice of equipment out there).
you are on the right track for testing for intake leaks, mass air flow sensor and plugs and wires. thats the first place i would have hit first too.
at this point i would be watching the missfire counters to pinpoint excactly which cylinders are acting up.
since we cant do this , we can only recheck and try a few things without having to pass the job off to someone with the equipment needed.
1. recheck for vacuum leaks.
do this by watching o2 sensors at idle and spraying cleaner while watching. if it goes rich, than you have a vacuum leak.
vacuum leaks are not always detectable by listening and rpm increase. the most accurate way is to watch o2 activity while spraying.
2. faulty o2 sensor
swap the front 02 sensors left to right and see if the .020 freeze changes to the other side.
if it does than the o2 is lazy and will need to be replaced.
3. check coils and make sure they can all jump a 1/2 inch gap with a nice blue spark. if any of them dont than replace them.
4. if 1 entire bank is missing, check for timing marks on that side and see if the timing belt jumped.
5. swap injectors from 1 side to the other if possible and see if the 02 freeze changes side. if it does than there is a bad injector.
6.clean and recheck throttle body and make sure there are no cracks in the air intake snorkel tube.
7. compression test