If you can understand a workshop manual and have some basic tools an engine
is no big deal..
filters is pretty straight forward and with practice can be done in minutes..
pump impeller is 8 screws on the face pull out the old push in the new screw face back on..
There's also tricks if your fuel
fails, blockage/whatever that will have your engine
running again in 10 minutes on fresh fuel.. a 10 litre jerry can and some hose direct to your fuel pump
for an emergency
These are all things one learns on the go..
I started enginless, then outboards and did not have a diesel
till my 7th boat
.. my first one over 25ft.
The reality is much less daunting than you imagine unless your all thumbs and two left hands.