Like I said, waste oil boilers take a lot of maintenance. Every few hundred hours of run time, perhaps once a month, maybe more often. Every situation is different. Different oils create more ash.
You mention about a 10,000 sqft building. Depending on which manufacturer of PEX tubing you talk to they will say roughly 25 to 30 BTU prt foot of tubing. In my experience, and I have done two car garages and everything up to 35,000 sqft truck shop, you will use very close to one foot of tubing per 1 sqft of floor. so 10,000 sqft x 30 BTU comes out to 300,000 BTU. No sense in putting in any larger boiler than that because that is all the tubing can handle. No if you add a side arm domestic water tank, or some forced air hydronic heaters you might want to add that load to the mix.
