You will need to pitch the exhaust towards the exit to prevent condensate from running back into the unit.
Also check outside, could be terminated in such a way that heavy rain and wind will drive water into the exhaust.
Plumb the combustion intake to the outside or you'll not burn efficiently...
The heater should be flushed annually depending on hardness of your water, possibly twice a year.
In order to do this effectively, you'll have uninstall the boiler drain valve and replace it with a 3/4" full port ball valve.
I replaced my several years ago and took a vow to PM it annually but...
There are 2 sets of thermal limits. Each thermostat is protected by a high limit thermal fuse.
It is recommended replacing both the fuse and the thermostat per set.
Youtube it, they have very simple videos on how to troubleshoot.
Look out for used parts wherever you buy and try to get OEM...
You need vents at the soffits and at the peak of the roof. You have to have the correct flow so that it does not flow backwards. A good roofer/builder will be able to tell you (calculate) how much/how many of each (soffit and peak) vents you need.
P.S.- assume your installing drywall over the...
Sounds like a situation where pre-filter wouldn't hurt.
You can try different MERV ratings while keeping an eye on your blower's amp drawn and determine which filter is best for your system.