Do you already have the tank/stand? If not I would consider a 150g instead of the 125g, same footprint just taller. There is also the 180 & 210g which is the same length just a few inches deeper. The bigger you go.... the easier it is to care for & the more cool fish you can buy
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Like mentioned what do you want to keep in there?? If a full blown reef then you will want to go a refugium route with lots of live rock (gets expensive) & you also need to add additives to keep all the inhabitants happy.
IF you want fish & only fish then a wet/dry filter is all you need, this is what I use on my aquariums. You do not need any live rock & zero additives are required. Lighting is also up to whatever you want it to look like vs reef tanks needing a lot of lighting to keep corals & everything else thriving.
Either way you will want a reef ready tank which has internal overflow boxes which allows for a sump to easily & safely be used under the tank.
A good protein skimmer isn't a necessity but it makes life for you & also the fish a lot better
Interneal or external return pump, heater, gravel/sand & that's about it for what you need
If you already have the tank & if it's not reef ready I would have it driled so you can install a overflow box in it.