Jeff Rapps at www.tangledupincichlids.com has a nice variety of Heros, probably not worth it to pay for shipping if you only want one though. You can find some good deals on www.aquabid.com sometimes too
Bed Bath and Beyond sells aquaclears?Seems I'm a little late, but I agree with most of what was said. Really pick what species of severum you want. Advantage of just getting one big, you know what it'll look like. Disadvantage is cost and you loss out on the fun of them growing up. Honestly corys and deep bodied tetras will be fine, especially if you start with juvie severums.
As for the AC 110, go for it! Great filters and with a bed bath and beyond coupon that regularly gets emailed out you can get one for about $65. I do always recommend running a sponge filter though so you can easily move to a sick tank at any point.
Festivum (mesonaua) are a great compliment if you enjoy schooling fish that are a bit more personable and pretty.
Cories will be fine as well as plants...just stay away From delicate stem plants. Deeper bodies tetras are fine as well. Rotkiels aren't notorious plant eaters from my experience. You can go with EB acara' sir you want them but holding two pairs of both when grown in a 75 is pushing the bio load.
Most guys at the LFS are pretty cool. Not all of them are as knowledgeable as we would hope. I've kept rotkiels for some time now and know them pretty well.