The 171-worked trains do always seem to get very busy - presumably because they look a bit longer and more impressive but are generally no more than 4 coaches long in the late evening period. If they're going to Uckfield, First is enforced - if just to East Croydon, most likely not, even if it might in theory be in the timetable. FWIW, a number of people get so confused about whether First is formally declassified or not on anything other than a 455, that it probably has just the same number of passengers as if it was enforceable.
There are a few First Class ticketholders to be found on those trains where it is enforced/not declassified, but, as above, demand is only really there on medium-distance journeys, and even then it really is very limited. You'll sadly be likely to find more fare dodgers or unsuspecting Oyster users in there, until they get chucked out (if that happens) which usually would not be until the train passes Sydenham anyway.
For such a short journey, I really would not worry about buying a First Class ticket. Save money and hop into Standard. No point getting worried about which trains have First Class or not.