Collectible is whatever you want to collect, period. Most collectibles don't appreciate in value and are not good "investments." If you like it and think you'd rather have it than the money, buy it!
I'd think this would be an okay acquisition IF you collect and display bare-metal O-scale locos. That's not what I do but it would be a good addition if you collected them.
It would look pretty good with the MTH French 241.A in bare metal (its almost exactly the same size - the S3 was a slightly larger loco in real life but the MTH is 1/45 scale rather than 1/48 which just about exactly counters that). The 241.A is the loco D500 was referring to above and worth discussing with regard to this S3. Bare metal tends to make all the added details and parts stand out more, and the S3 only has a medium amount of them. The MTH 241 has a lot - it looks like the jewel it is in bare metal or paint. So the S3 will will look a bit bare sitting on a sehfl next to the MTH 241 or some other premium unpainted brass or similar locos. Still, its an impressive model in bare metal. More than respectable.
By the way, some people including MTH refer to the MTH 241.A as a "Chapelon loco" but the model is of the loco after 35% at most of Chapelon's modifications were completed (among other things, when it was complete, it had a two-axle rear truck, making it, in French nomenclature, a 242.A).