Yeah, no problem!
With the BioCubes (and things like them) for saltwater you generally want to upgrade some of the things, adding a heater and replacing the filtration and lighting usually comes first and then you can do whatever else your budget allows afterwards.
Since you want to keep a purple firefish, you will need to keep your lid on. Generally building them for a curved tank is very hard so you don't want any lights that have to be outside of the tank. A DIY retrofit kit is the easiest way to do this. If you're not going to keep corals, then upgrading lighting is useless but you can only keep softies/polyps, and anemones with the stock lighting. They also sell cheap little LED
strips on marine depot that work pretty well and you can better choose your layout. Retrofit kits are usually a little over 200 bucks and the strips are pretty small but like 20-25 each.
To upgrade the filtration, I'd rip out the bioballs and buy a media basket and fuge from InTank (like 90 for both) and set those up with whatever media you like best - purigen is a good one to have. You can add in a protein skimmer to hang on the back too, I definitely recommend this. Those are usually around 100 bucks.
Cobalt MJ1200 makes a nice return pump. Not sure of the price but under 100 bucks, maybe even under 50. If you want to keep SPS
it's necessary. You can also add powerheads - Hydor Koralia Nanos are good and there's no need for anything super expensive/big like the MP40 because that's a TON of flow.
Cobalt makes a nice heater, I think it's the neo-therm. Go with a 75 watt or very close to that.
You could go with an auto top off system but that's pretty expensive and unnecessary.
With the bio cubes you get a built-in sump with a volume of around 5 gallons so just upgrade that, add a skimmer, some live rock ,and you'll be fine. Oh, and a clean up crew too. I don't recommend a canister filter because they cause water to go really bad if you don't thoroughly clean it every day or so. Buy base rock and seed it with some live rock rubble, it'll be WAY cheaper than buying 40-50 pounds of live rock. I would buy the rock in person so you can see how big the rocks are because 50 pounds of one type of rock is enough to fill a 125 gallon, whereas 50 pounds of another is good for the biocube or similar.