New project 17 inch cube tank

There was a time that 18" cube tanks were 'the' tanks for Discus breeding, I still have a couple and also one which is something like 18"x18"x22" deep which developed a leak and was dismantled pending a rebuild. I used to keep some small Angels in it as a 'growout' tank before it started to deposit its liquid content on the computer-room floor.
I also have a 22" cube tank (which might actually be 23" cube) which contains my Bengal Blue Snakeheads. They've grown to the sort of size now where they're a bit less tolerant of one another, but so far it's just posturing.
For what it's worth I think you might be 'over-egging the pudding' to use a sump, but probably others will feel different. I guess it all depends on what inmates herself would like in her tank (if memory serves me right Catherine likes Angels? The tank would, even with a substantial-sized sump, be too small for an adult pair of Oscars, but would be OK to grow them until the inevitable differences-of-opinion start up. You would need to build a hood/cover for it, possibly 2x15" light units would be sufficient - or you could look into using the LED options so well-described by Bart but you would, I reckon, be able to get by with two - or even (at a pinch) one unit.
Maybe you might think of starting up a planted tank - that would be a nice size - with a reasonable depth for some plants - but, of course, the final choice would be that of the two of you - these are mere suggestions, doubtless others will be able to make other suggestions also.
It will be interesting to see what you finally decide on.

John