I worked on some old boiler, they are not efficient and they cost a lot for the repair. but I saw some new are very good but expensive. in my opinion, boilers are not right for real estate investors, let me know,  follow up I will do my best to help you on that. have an HVAC Tech to look at just for a regular check-up and he will tell you.

keys cost a couple of dollars. 

Yes it may cost a few hundred dollars but so will a lawyer. it just depends on how you want to spend the few hundred dollars.

I'd contact the Contra Costa County Public Administrator's office: https://www.contracosta.ca.gov/962/Public-Administrator  You might need them to take over and look for a will among the belongings, etc. 

Plumbing quotes always cost a lot relative to the work (in 99 percent cases).

I think it's far more likely that someone slammed or kicked the door, but it's your house and you can make that determination.Second, it's an old door so you shouldn't charge full replacement cost. A

I did a cost analysis and found I can swap out all the window ACs in a house (a 10K central unit, two 5K smaller units in bedrooms) four times, labor included, for the same cost as swapping a central AC system.Keep in mind that central AC is not maintenance proof. 

It’s not going to cost a service member an arm and a leg to get back their money.

Yes, they cost a monthly fee but there's no brokerage you need to abide to or yearly dues.

If I am doing the work myself, the repairs and preventative maintenance will likely cost a lot less.