Unless you have a dedicated studio space you know well, I'd start with the A5x and add a sub down the line if you find you need it. If you buy a modern audio interface, as long as it has good drivers with your computer (pretty much impossible to go wrong with Focusrite) you'll be set. They all use pretty much the same A/D and D/A and very similar amps up until around the $500 price point.
Basically nowadays the D/A converters even in entry-level interfaces are good enough so that there won't be any serious drawbacks quality-wise in, well, almost, anything*. So no - the interface won't be a bottleneck. Choose one that has the i/o that suits your needs and one with good reviews (although this can be hard to judge - many people seem to be five-star happy just because they have an interface or simply don't have serious experience about different interfaces) and drivers, and fits your budget. Focusrite's Scarlett series is an easy middle of the road choice that's extremely popular (I'm using one myself at the moment). But unless you're going super high end, there's no need to think about "pairing" monitors and interfaces.
* There of course are $30 interfaces – I've no experience with those or knowledge which converter chips they use, but speaking of any well-known brand this shouldn't be a problem