considering you like the oddball fish (elephants) etc i would highly recommend the reasonably hardy "Polypterus ornatipinnis" or ornate birchir. Very colourful if you get a decent supplier and not too expensive. Great character too.
I'd also consider keeping the angel display tank you were thinking of. Although a little boring to start with they require roughly the same water conditions as Discus and so upgrading to Discus later would be reasonable. Once you have the angel display at its optimum (perhaps dabbling in breeding) you could go for Discus at minimal expense and upheaval. Might consider and RO unit in the future with Discus them too. A second advantage to the Angel display is that once you are more confident in your water quality you could add your elephant nose. Some debate on how many to add but I'd go for one and add another 3-4 a fortnight later. Remember that your experience is constantly growing and your tastes will change - angels.... with elephants... then discus later on when you're more confident???
Have you considered black ghost knife fish - beautiful (but not to be mixed with elephants).
I own the Tetra EX700 on a Rio 180 and have loved it. Maintenance is a breeze and it has run for almost 2 years without a problem. Mechanical filtration is superb, which is what you want for the larger show fish. Would definitely recommend the EX1200 for your Rio 300.
Good luck!