Ah, my kind of question, hope I can help! If I use any terms you're not familiar with, don't sweat it, you'll enounter them soon enough (do a search on this site and you'll get tons of info).
Think of a sump as you would the back tank on a toilet. Has the same water as the "business end" and serves as a "reservior". Water continioulsy flows out of it into the "potty" bowl. In a sump, however, it flows in AND out from the main aquarium.
The sump is basically, also, a giant filter box. Done right, it's the best thing to have on a tank. You can put a ton of filter media if you wish, activated carbon, protien skimmer, calcium reactor, etc.... anything you want. Put your heater in it. Dump fresh water in it to top off evaporation rather than dumping it in the main tank. Add medication or additives to it. Dose kalkwassar to it. Put a bullied or bullying fish in it as a form of "solitary confinement"
Add lights to it and grow macroalgae in it for nitrate/phoshpate removal. Or build a "refugium" with live sand and rock and macroalgae and connect it to the sump. Lots of options.
As for the fish, in a 75 you have a lot of options. I'd stick to mostly dwarf angels, and even then only ONE per tank. *MAYBE* two if they are similar sized and introduced at the same time. Most other angels get bigger than a 75 can accomodate. Do research before dumping $60 on an angel, some get to be gigantic (relatively speaking).
Clownfish are a breeze to keep, note that I've had bad luck with them jumping out and freeze-drying to my carpet. Odd, b/c I've never heard of it happening before, and my water parameters were squeaky-clean (Ie, no polluted water causing them to try to escape).
One last note, and I do hope you take this seriously and heed my advise, for the sake of your wallet and for our natural resources:
Corals first, *then maybe* an anemone. The anemone is a nice thing, but they really do require special attention. They do need to be fed every other day or about three times a week, *very lightly*, and they need good light (at least 3.5 or so watts per gallon for most). There are only a few that host clownfish, not all will accept a clown or vice-versa. And overall anemones are much more picky about water conditions that most corals. I'd avoid the anemone until about month 6. By then, the tank is fully "broken in" and established, plenty of microscopic life flourishing, and water is well-established with many beneficial bacterial populations.
Sorry to dissapoint you on the anemone thing. I killed three or four of them b/c I didn't do enough research. Oh, I did some. Just only read the advice I wanted. Had a heeded the good (ie, disappointing) advice, I would have killed four less anemones and saved about $80-- that buys a LOT of sea salt!
Oh, and for a reef tank, use a good brand "reef" grade salt. NOT "Instant Ocean" sea salt--- it's fantastic and hard to beat for fish-only but severely lacks in the calcium content. I use Oceanic. Good stuff. Only slightly more 'spensive than Instant Ocean. I've tried Coralife and hate it. My corals didn't respond well to it at all.
