If you we're to go with the buildmyled.com you could keep some carefully placed corals down the road. You would need a dimming kit (only $15 extra) to dim the fixture.
While diatoms can be a sign, i would not say it was cycled without water tests confirming. Even if it is cycled, it is far from stable. Again, it's good that you have a large amount of water to work with but don't push it too fast.
Can't give you a timeframe on when the tank will be ready. Water tests are the only way to know. Two weeks is likely the minimum though. Once you do start stocking fish, test afterwards and don't add more until your sure the tank can handle more load.
A uv is not a bad idea but you should feel that your tank is bulletproof from parasites just because you have one. I personally would only run one if I had multiple tanks plumbed together. I don't have enough experience with them to recommend a good one. They do make your water clear though .
An ATO would be good investment for you. Just make sure those floats will fit on your tub.
What's the best place to buy fish online? I've used liveaquaria and saltwaterfish.com before, any other good ones? My local fish stores don't have a lot of variety.
Qualitymarine.com has a lot of different species and types of fish not seen on other sites but I guess you must be a fish reseller and have a business license to buy from them.
Liveaquaria is the best IMO. Their Diver's Den section is very good. You can sign up for a daily update of the fish they add each day. You will get a preview and then the actual update of what was added later that day. LADD is a separate facility where the QT all the animals until they are eating and have been observed to be in good health. They also offer a 14 day guarantee.
LADD gets my vote as the best but Blue Zoo Aquatics is also a reputable fish retailer.
If your LFS is good other than selection and your want to support them, research and find what you want online and see if they can order it for you. My LFS is excellent and can order just about anything. They also will hold the fish until it is eating and looks to be in good health. They will also QT in copper if needed or requested. Always good to develop a good relationship with a local retailer. They can be a lot of help and will be there for you in a pinch.