1) Lower level player population is very low so it's harder to consistently find a group under level 70. You can try the booking option or find a guild and level with their alts (alternative chars). Most players don't stay under level 50 for more than 1-3 days because they know where to hunt/quest, and how to gear/skill, and have VIP/turn in materials for exp quests. For solo play, bounty boards are your best option, IMO. Use iRO Wiki to see what you can or cannot hunt at your level, and where to find them in large quantities.
2) There is no easy zeny unless you are high level because most commonly used items in high quantities are bot farmed. To get started with a little zeny you can farm Stem from Mandragora (10+), Strawberry from Spore/Boa (10+), Poison Spore from Poison Spore (lvl 25+), and sell reliably to other players. Maneater Blossom from Flora can be farmed when level ~50. Bounty boards qiests will award zeny. Use ragial.com to see if the item has higher than NPC sell value before dumping it at vendor with Overcharge lvl 10.
I think Merchant is a difficult class as a first char. You'll be using lots of the novice red potions, and I would suggest doing the Eden quests that reward them or you will find yourself sitting often between fights. My first char was also a merchant, and it took a lot longer than thief/archer (common first chars). Good luck!
