I've had a couple from this era. They are glorious boats. 472 cubic inches with a cam that tops out at 4500 rpm means torque levels reaching insanity. These often came stuffed with 2.43:1 gears and still roast tires. I suppose if it's a late 69, it could be a 500, but they weren't typically installed until the 70 model year. Only way to know for sure is to measure the stroke with a screwdriver or welding rod. 4" is 472. 4-1/4 (4.304) inches is a 500. Only difference is the stroke.
The engine will last forever. I had a 500ci caddy with well over 500k on it before it started smoking. A teardown revealed a cracked oil ring, but it still had crosshatches in the bores. Super-high nickel blocks means they're hard as granite, but if you try to up the ante (over 600hp) they can get some "sing" to them. Factory pistons are cast, and there aren't many aftermarket options without going custom.
The RWD cars were based on the GM B, C, and D bodies, which are nearly identical except for minor differences, so think like Olds 88/98, Pontiac Bonneville, Chevy Impala/Caprice, etc. The FWD cars were E-bodies and the only other E-body option was the Olds Toronado. Fewer parts around for them, but still available.
Transmissions are parts-bin GM. Most of them came with a TH375, which is a "softer" TH400. If it's FWD like an ElDorado, it's a TH425 which shares a lot in common with the TH400 as well. Bulletproof, and cheap to rebuild. Same bolt pattern as Buick/Olds/Pontiac, so cores are a dime a dozen.
Even if you're not planning on making more power, a swap to an Edelbrock intake (yes, they still make them) will pay big dividends if it fits under your hood. Factory Caddy intakes were cast as a super low-rise intake to clear hoods with it's massive deck height. The intake charge goes into the plenum which is below the ports in the head, so it has to go down-up-down. That causes a bit of fuel to come out of suspension. Swapping to an Edelbrock not only frees up about 15hp, but you might see better MPG.... which actually isnt' bad to start with. I was getting 18 mpg in a dually with a Caddy 500/TH400 and 3.73s. Another hack is to get an HEI from a later caddy. Way more spark energy, and no more points to file and set.
If you want headers, the easy button is to make or buy Caddy flanges, then use BBC headers. Cut the flange off the BBC headers and weld on the caddy flanges. Bore spacing is within about 1/8", so it's an easy tweak.
I never had trouble finding parts for my 62 Caddy Deville, but that was 15 years ago.
Just do it. The only problems they tend to get are dumb electrical stuff from all the options. The good news is that they're simple. Modern high-option cars have a thousand wires that disappear into a magic computer box. Old Caddys don't. If your heater blower motor stops, it's either the switch, the fuse, or the motor. Diagnosing that on a modern car is more like a 12-page flowchart of "test for voltage at BCM pin 31" then "verify coolant temp sensor input range," it gets nauseating.