1. Current gameplay mechanics minimize the need for those old school selective linkshells. You will likely be able to find a linkshell on any server that has a mix of veterans, newcomers, and returning players. End game content is usually instanced (such as entering a battlefield) or pop-based. The days of camping against 250 others for Fafnir or whatever HNM you choose died out a few years back. I find the vast majority of linkshells are social.
2. All jobs are incredibly easy to level for multiple reasons -- be it mage or melee. Even as a bard main you can DD your way through to 99 if you prefer. Look up "Trusts" for more information. However, once you reach higher levels there are more efficient ways of leveling quickly such as Abyssea. The amount of changes to the game are way too complex to be summarized in this response but you will easily encounter people in game willing to point you in the right direction. It's not nearly as overwhelming as it seems initially. There is really no being left behind in terms of levels these days because it is so easy to level. Also look up Grounds of Valor and Fields of Valor in relation to super fast leveling.
3. All jobs are cheap to level (not including purchasing spells and the like). Ideally it would be best to level a melee job that does not use consumables. You can save up gil while leveling and eventually find your niche to make gil. You will level so quickly on mage jobs that I wouldn't recommend it to a returning player unless you are really fond of questing and finding means to make gil early-on. There are various cheap options to gear yourself.
First of all, you will level so quickly and easily that you will outlevel your gear within the span of a few hours in one session. For this reason, many players including myself see gear as *nearly* pointless at early levels. The AH has a combination of a dearth of choice (due to people having no interest in listing low level armor and weapons) but also a surplus in terms of low level armor and weapons that are used for crafting, etc. It is normal for example, to find some level 20 gear, do some leveling, and not find any gear worth your while until you reach 50 or so. Additionally your Trust NPC's will make up for your shortcomings. It is important to note even if you desperately want to "do it right" you may not be able to due to AH constraints and not yet having sufficient craft level.
You will level so quickly that you may choose to level your "subjobs" to 49 or so. You need to experience it for yourself to realize just how easy it will be to level your subjobs to the appropriate level for your mains.
4. Level up all jobs you enjoy and want to play. As emphasized, it is so easy to level them now. I might also add here that Records of Eminence allows you to easily obtain iLV 117 gear without hurting your wallet. This gear is more than good enough to see you through "catch up" content such as CoP missions and the like which are now all more-or-less solo-able. For a lot of the high level content the herd mentality prefers PLD's, well-geared MNK's, RNG's, and BRD's. However it should be easy enough to find friends who will try high tier content with any job you choose. I don't really see anyone playing DRK hardcore these days, but that's just my personal experience. Another job which hasn't gotten much love from SE lately is NIN. Hopefully a main DRK will have more to say on this.
5. I left FFXI for a while to play FFXIV. I lost interest really quickly. I feel it doesn't bring people together socially the way FFXI does. However, the recent changes to FFXI seem to be an attempt to make it play like FFXIV. I recommend just trying it for a month to see what you think of the changes. While I'm not economist, the fact that the game is more than a decade old and that it is still $12.95/month means it is cheaper to play than ever after factoring inflation. You don't even need to buy an expansion pack to enjoy the fields/grounds of valor content and the like, so I think with just about 2-3 weeks of gameplay you will have made up your mind quickly.