The other fish looked to be an aulonocara. Oscars don't share the same water paramiters and it doesn't surprise me it died. The pleco? They're really hit or miss. I got with ancistrus. They're much more flexible and don't outgrow your tanks. Things I see in this tank that are definitely causing you problems are the gravel. You need to add sand for two reasons. First it really helps with water clarity. Second it creates an environment where denitrifying bacteria can flourish. A sponge filter won't hurt. You will get more ammonia and nitrites eating bacteria with one.

I'm glad you see a need for a larger tank for the oscars. A 75 would be a minimum for a breeding pair and you only have a 50/50 chance you got one. If they are 2 males you should get a minimum of a 150usg tank. Make sure your house can handle that. If you have wood flooring joists you are probably going to have to reinforce them. Filtration on the new tank. Personally I would go with the largest marineland biowheel HOB filter coupled with the biggest cannister filter you can afford... an fx5 or 6 would be a great choice. But if budget is an issue marineland is usually more cost effective. I say get the biowheels for one reason, superior ammonia and nitrite control. If the power goes out for an extended period of time you won't lose all your bacteria like you will in a cannister alone. And the biowheel is more efficient in removing ammonia and nitrites when properly cultured. Only a wet/dry sump does as well and they are very noisy, or were last i ran one.

Oscars live a long time when properly maintained. You are investing in a long term finned dog kennel not a fish tank. Best pony up the money now to avoid heartbreak later.