The watch on sale has already been serviced. But yes it will need service again in future. However, an automatic chronograph is always complicated to service, whether it is a Seiko or a Zenith. The Zenith is a little more difficult, but there isn't so much difference in actual work. I think in both cases the money is well-spent, but the Zenith may be more worthwhile.I'm a big fan of the Zenith El Primero and would love to own that watch at some point, but the used market on these watches doesn't seem to be favorable. The original design is still the best one and is the one to get, but used ones are so expensive, and then servicing has to be factored into the equation because that watch is considered to be very difficult and expensive to service -- and that's if you can actually find someone to work on it. I think owners of used El Primeros have gotten a lot of traction out of Zenith's grossly inaccurate claim that this watch was the first automatic chronograph; third is more like it. Making an announcement that you have the world's first automatic chronograph without actually making that watch available for sale doesn't mean that you have the first automatic chronograph; it means you were the first to make an announcement that you're going to be selling an automatic chronograph. Heuer and Seiko actually have much stronger claims to being first.
In any case, and given that history, I don't see the value in spending more money on a used El Primero when a new one with the original design can be purchased on the grey market for under $6k.
The Zenith El Primero was the first automatic chronograph to be presented to the public on 10th January 1969 (Zenith already had working prototypes in 1968). The actual serial production by Zenith was slower than the production by Seiko and Heuer. Seiko was the first to achieve actual serial production of their (simplified) caliber 6139.
However IMHO it is not so much because the El Primero was "the first" automatic chronograph, but rather because it is the best, that it is so much appreciated.
Yes, you can now buy a new Zenith El Primero with similar design (not the same) for less money. But you will then have a new watch, which the moment it leaves the shop and stops being new, will loose economic value. The original one on the contrary, although it is already used, may continue to gain value, because of its historical value and because it is a supreme collectible watch.
However I don't think one should get it for the economic value, and not even for its historical, horological value, but rather because you love it. And if you don't, don't. ;-)