I dont have any information on server populations but as someone who had a level 75 that got deleted way back I can tell you that getting your characters levels back will be much easier this time around due to the alliance parties with grounds of valor pages. Gusgen Mines and Crawlers nest both usually have people kicking around in them and you can get to level 60 just doing those, although there are alternatives. Ive yet to do Abyssea as Im working on limit break 3, but Ive read that once you get into it you can gain levels very fast there as well. Theres alot of stuff to do both solo and endgame wise and once you reach 99 you can beat most of the missions by yourself because they removed the level cap for alot of the fights. I would also highly suggest looking into leveling the Dancer job because it has a ton of useful abilities that make staying alive much easier. Im working on Beastmaster and without the dancer subjob I would surely have not been able to complete some of the missions/quests I did at the particular levels I did them. If you have any more questions I'll check back, goodluck.