As for the construction of the stand, it should hold up to anything you can put on it. The biggest problem you face visually is the three very different types of wood used for the exposed areas (white pine, oak and maple). You may be able to match the colors on all of the surfaces by using different stains on each one. One single color of stain will appear different on each type of wood. You might be able to use certain stains straight from the container, but more likely you will have to custom mix three different stains to get a match with all three woods. When I was doing woodworking in Florida, we had to try to match the "zebra mahogany" paneling for a project on an early 70's Hatteras yacht, but this was in the 90's, and zebra mahogany could not be obtained from any source--it simply was not available anymore. We did some experimenting and ended up closely matching the grain effect with teak paneling, which we had to bleach and then stain to match the color of the mahogany. A roundabout way to get there, for sure, but the result would stand up to all but the most knowledgeable and experienced of inspections--way better than good enough to pass under even more than casual observation. It would take a wood expert looking for it, to see the difference. Unfortunately, your challenge will be greater, because the grains of the three different woods are so different that making them "match" is far beyond getting the colors right. The oak is going to be the one that stands out most from the other two; even though the maple and pine are very different woods, with enough sanding of the white pine, its grain can be minimized to more resemble the fine grain of the maple. The color may be easier to match between the two, as well. The oak, on the other hand, is very distinctive with its deep, coarse grain and richer color, and will probably stand apart from the other two woods regardless of what you do, aside from filling the grain of all three and sanding smooth for a opaque lacquer look (which is probably near to the look your old, cheaply made stand was an attempt to emulate, and you didn't like it). Other than that, your best bet is probably to intentionally make the oak stand out from the other woods, possibly by leaving it with a more natural color stain while using an actual colored stain for the two whiter woods. I have seen some colorfully stained woods that looked absolutely gorgeous with their natural grains showing through rich green, blue, or even red stains. A dark silver or light charcoal grey would look good against the natural oak, as well. If you want to experiment, just remember to start with your lighter colored choices and move to the darker ones, keeping in mind that the next color of stain will have to either cover or blend well with those beneath it. Also, since we are talking about an aquarium stand, after you get the color scheme you want, finish it with something like you would use if you were going to stick it out in the yard, in the weather. That way, when (not IF!) it gets wet, it won't easily show water damage.
...Also, is the stand strong enough without a central upright?
That stand looks very strong, even without a center brace. Most aquarium stands that have a center brace are made with materials of much less strength (MDF or 3/4" very light and soft wood) than this solid 2"X~ wood. Most aquariums are actually supported by their corners/ends, anyway, and you can often slip a card beneath their frame in the middle span. The real tank-killer is twisting or torquing; as long as all the corners are on the same plane and every part of the support is even with or below that plane (no high spots), most any aquarium under 55 gallons should hold up like a champ. (And probably larger ones as well, but when you get over 75 gallons, that's a lot of weight and pressure to keep in mind, not to mention the bigger consequences of any lapses in judgement.)