Congrats on the car and the great progress that you are already making! The body, frame and interior look great! Judging by the condition of the seats, it appears like a low mileage car.
5052's brake fluid looked about as nasty after 20+ years. Without a doubt, replace the master cylinder, all four hoses and rebuild the calipers. DeLorean brake hoses seem to develop clogs after long term storage. Furthermore, the crud will likely plug the internal passageway of the front calipers. It may even clog the brake line between the two halves of the rear calipers. In contrast, I have never seen the metal line itself clog. (However, I'm sure it has happened to somebody.) I suspect that the clutch master cylinder is just as bad along with the hose from the supply tank to the master.
I've never seen jelled coolant. 5052's coolant was still liquid, as was the coolant in a DeLorean that I recently looked at that had been sitting since 1986. I wouldn't pull the heads. However, I would recommend draining the cooling system, replacing all the hoses and the thermostat and refilling the system.
The fuel in the tank actually looks better than what I would have expected, as it is still somewhat liquid. 4194's was a brown liquid after 7 years of sitting between 1991 and 1998. 5052's was like road tar after sitting from 1991 to 2014. Be careful with the rags that you use to clean the tank. I don't know if they can spontaneously combust.
These cars seem to get parked for any number of reasons. Often times they get parked due to other demands on the owner's time / health issues. Other cars get parked because of a minor or a major mechanical problem that the owner does not have the motivation or money to fix. Lots of DeLorean's got parked early due to a belief by the owner that a DeLorean with low mileage would appreciate in value with the passage of time. In the case of 4194, after spending a number of months getting it running back in late 1998 to early 1999, I discovered that the car likely got parked because it needed a clutch!
Keep up the great work!