The tank you want to build is too small for one oscar, let alone a pair. With your dimensions, its about 67USG nominal and about prob 60G or so with a normal water fill. You need atleast 90G at a minimum but I would say atleast 125G or more if you want other tank mates....
For more than one:
....if they are a pair, atleast 150G to 180G (and remember, the only way they are a def pair, is if they have mated before. Otherwise there is no guarantee that you won't find one oscar messed up in a corner one day). If you want a pair, buy 5 or 6 little oscars and you will be able to see after awhile a few of them hanging around eachother. Keep the two that look the closest and maybe they will grow into a pair one day, but don't be dissapointed if one day they fight and you have to get rid of one, cause it is a def possibility.
.....if they are not a pair...ie. two males, you will need a large tank, prob upwards of 250G top 300G for males and atleast 220G for females. They will need their own territory that is divided up by plants, rocks, wood, and/or other decorations. Anything smaller and you will have a constant brawl in your tank.
Would it need to be heated if the temperature stayed above 72 degrees?
If you create a fish room and keep the temp at about 76F to 78F degrees you wouldn't need a heater...otherwise, your gonna need one to keep the water around 78F.
Honestly too, it would prob be easier to just buy a tank and then build a stand and hood. If you make the tank, your not gonna be able to make a tank as good as the manufacturing line of a tank maker can.... plus it'll prob be more expensive to make your own due to the cost of acrylic or glass.
