RobbieY - It would be better to start a new topic for your question. Regardless, I will jump in with my thoughts.
Its been 18 years. A cheap roof may only last 20 or 30 years, maybe even less if it was a real shoddy job or has a low slope.
I cant say for sure if new flashing would have been required or not. I do know that tar, caulk, spray, and other coatings are not proper long term sealing/repair solutions.
If indeed you need flashing and do not have any then it must be installed. The shingles adjacent to the flashing can be removed, flashing installed and those shingles replaced. A decent handyman or even a competent do it yourselfer may be able to do the job. If you contact a roofer they may recommend replacing the entire roof. That recommendation might be legitimate, might not be.
No sense selling a house just because the roof needs flashing. Closing cost alone would be more money that the repair. Do you attend church? Maybe your church has a Deaconal board with someone handy who could help you out. Do you have any friends or neigbors who have done roofs before? Friend of the family who is a roofer who may want to do the flashing job in exchange for the cost of the materials and a couple of apple pies?
Remember - if you have a leak then the water is causing more damage and future repair costs will be higher the longer you wait to fix it. if its been leaking for 18 years you could possibly have some serious damage you cant see.
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