I posted the link above because it focuses ALOT on the importance of the business plan, etc etc etc.
If you haven't done so yet, read Birdsill's columns - he's been there and done that, and has alot of great insight. It makes a great "LFS for Dummies" (or at least the first chapter).
Plan on a fair amount of overhead in drygoods - For example, even if you specialize in SW/reef, you'll want to carry a line (or more) of "classical" hang-on filters (for FW customers, or Fish-only customers, etc) --- Whisper, Aquaclear, Penguin, etc. You're going to want to have 4-6 of each model on hand, along with cartridges, refills, medications, airpumps (etc), nets, food (lots of foods to choose from now), lighting, etc.
If you add in good quality skimmers (euroreefs etc, not seaclowns) , high end lighting (MH ? PC? T5 ?), sumps, calcium reactors, etc - you just easily tripled your dry goods inventory cost, maybe more.
Then there's the livestock systems - are you prepared to design and build a system yourself ? Or are you going to be using "off the shelf" units like the Marineland MARS units ? Or simple individual tanks with UG filters

? Theres a cost associated with each, and that cost is a blend of initial time (pre-open), install time, purchase $, operating $, and maint time. Some will cost you more in one area, some will cost you more in others.
15 years ago the store I worked for dropped over $40,000 on a 48ft unit of livestock tanks (3 level rack, plumbed into huge wet/drys with UV etc, all acrylic) - I don't know what a comparable system would cost today.
What clientele are you looking for ? Is this a fairly affluent area or more blue-collar ?
(I just saw a good local store go under after about a year, and I'm convinced they didn't take their location and customer base into consideration -- they decorated the fishroom in teal with brass, very high-class look, but pricey. They were charging about 1.5-2 times what the chain stores were charging (for everything) - and they were in a fairly low income area. There just wasn't a demand for $8 odessa barbs and $100 bigeye squirrelfish

I think if they had focused on lower cost fish and spent less on decor they could have found a niche and then brought in the higher end items once they had a stable customer base paying the lease )
I was a store mgr for a LFS for years (fish dept mgr before that) -
generally we barely broke even on livestock (but we didn't mark up very highly) - mostly the livestock got people in and out the door, and they had to walk past the rest of the store to get to the fish (lots of impulse buys on high margin items).
That was before the internet (early 90's) took off, and before walmart became the number one seller of tropical fish in the usa. Now the margin on supplies is pretty meager (at least around here) so its a balancing act.
If I hit the lottery tommorow I wouldn't open a LFS. I
might start breeding FW fish and culturing corals semi-commercially, and maybe selling drygoods online. If I could build those up enough, I
might setup a small boutique-y shop in a high-income area.