To answer your questions as best I can

1) Either CUL or ALC. These two make consumables which is great if you are looking just for quick and easy sales on the marked board.

2) I use FFXIVGuild for leveling guides and this guide for everything post 50

3) Honestly, even with specialist you will want to level every class to at least 15 for the basic cross class skills. This is pretty easy to achieve and not too time consuming. The best cross-class skills are the lvl50 ones though and you will need these for high level crafting.

Buying mats is fine I guess, it's just not as cost effective. But both ALC and CUL need a lot of random things so be prepared to have a full retainer