Arowana is a no, tank is too small, they need a 180g or bigger.
Discus need pristine water conditions and may not be the best for a beginner until the tank is totally stable.
Oscars, can be great to keep but I would only keep one in a 125g as they can be very aggressive with 2 or more fish unless you buy a pair.
Loaches, I would perhaps base your stock around the Clown Loach, makes for a colorful tank with a school of fully grown ones, they get large as well (12").
If it was me, I would look at Lake Malawi cichlids and consider Placidochromis phenochilus, or Protomelas spilonotus
as the "show fish" with yellow labs(Labidochromis caeruleus
sp. "Acei" (yellow tail), and some Iodotropheus sprengerae (Rusty cichlid).