I wonder if I should use Omega or Alpha account for trading? Will the 1.8b/month cost of Omega subscription be too much compared with the potential profit of the autotrader?

Omega is the only way to make trading profitable. Also you need perfect trader: Brokers Fee at least 1.2% and Sales Tax 3.6% in Jita. About funds I would start trade with at least 10 bils. Otherwise trading will be less profitable then abyss hunting. I would follow path:

1 month courier till standing to caldari state and caldari navy 8+, 3-4 months of abyss hunter till your wallet reach 10+ bils and you level some trade skills, then autotrader.