Welcome to the hobby! In a few short months, you will be sucked into it and not have time or money for anything else!
I won't even try to answer all of your questions, as most need more info from you to give you an informed response.
Keeping in mind that you want to keep it on the unexpensive side, you should probably stick to a Fish Only (FO) or Fish Only With Live Rock (FOWLR). A reef tank will cost you hundreds and hundreds more. Keeping a FO or FOWLR tank is only a little tougher than keeping a freshwater tank, so if you have any experience there, you are on your way already! As far as the least expensive, FO is by far the way to go. However, LR has tons of advantages and IMO is worth the money. But, LR will cost a MINIMUM of 3$/lb, and the recommended amount is anywhere from .75 to 2lbs/gallon so you can see how it would add up quickly. You can always just put a few lbs of LR in too, instead of 50lbs+, but the more the better.
Oh, the blue light is an actinic bulb, designed to put out the light spectrum that corals and fish are exposed after the water has filtered all the light but the blue light.
As to what additional equipment, you will get many different opinions, however, bare minimum is a protein skimmer, I'd get one rated to filter around 90 gallons (again, more=better). Skimmers remove dissolved organics in your tank before they can decompose and create harmful chemicals. You will also need something that does more biological filtration (a place to grow bacteria in the water that turn your bad chemicals into less bad chemicals) This is most commonly done with a wet/dry filter and/or with LR. You could also use some more circulation in that tank. Get a few powerheads and position them in your tank. You are trying to eliminate "dead spots" where there isn't much current. It is in those spots that bad stuff grows. Beyond that, it depends on how hardy of fish/invertibrates you get. Equipment can be very specialized to the needs of the tanks inhabitants.
How many fish? Again, very dependent on which fish you put in. As a very general, almost useless rule-of-thumb, have a max of 1 inch of fish per 5 gallons of water. Giving you about 11 inches of fish. Until you learn more about the specific fish you have in mind, you probably want to stick to this.
To get you up and running with a FO will require at least:
75$ protein skimmer (over the back style)
2-3 powerheads @ 20$ each
sand - 4$/50lbs if you can find argonite based sand at a home improvement store
wet/dry filter - don't know cost, never bought separately
your lights now are fine, as long as the bulbs aren't too old >1 yr
test kits for - pH, phosphates, ammonia, nitrite, nitrate, alkinity
Salt - i'd get 150 gallons worth of salt. Its is cheaper and you will use it soon enough. around $35
A good heater, probably can get away with one 250watt heater.
Food (type depends on fish)
To make a FOWLR, you also need the LR (duh)
The cheapest way to get LS is to buy a Detrivore kit (bag full of little sand animals) or add some LS to your normal sand and wait for the animals in the LS to reproduce.
You MAY be able to get away without using a wet/dry filter if you have enough LR/LS, but I personally haven't done it.
That's all I can think of right now, I'm probably missing something, if I am hopefully someone else will catch it and let you know.
If you wanted to go Reef eventually, your biggest additional cost would be the lighting. Right now you probably have 2 40watt bulbs, giving you 80w total. Conservatively, you need 200 more watts to keep coral/etc happy. You would need to go with Power Compact (PC), Very High Output (VHO), or maybe even Metal Halide (MH) lighting systems. The cheapest way is probably the PC's, but that'll still run you at least 200$
LEARN ABOUT THE CYCLING PROCESS if you haven't already. I'm not going into it here, but it is vital that you know it well if you want to keep things alive very long. Do a search on this forum for "Cycling" and you will have more info on it than you know what to do with.
Best wishes, hope I answered some of your questions, I'm sure I gave you more than I answered, but that's the way life is
Feel free to ask me more, I'm happy to help.
Kris
