How many watts would this be

That particular brand has not had good reviews, from what I have read. It may not be a typical T5HO light. Typical T5HO lights can't be judged by watts per gallon. They light up the area under them to a pretty consistent light level, depending on how far the light is from the area where the intensity is measured, no matter how many gallons of water the tank can hold. A standard 75 gallon tank is 48"L x 18" D x 20" H, the same as a 55 gallon except more front to back depth. One single T5HO bulb can give enough light on a 55 gallon tank to grow just about anything. For the extra 5" in depth the 75 gallon tank has, a second T5HO light can give a more uniform intensity from front to back of the tank, but will also give significantly more intensity at the middle of the tank. For example, a 2 bulb Catalina fixture will give about 120 micromols of PAR in the center of the tank, and that is in the high end of high light intensity. But an Archaea 2 tube fixture gives about half of that - comparable to a one tube Tek Fixture. I suspect the Oddysea fixture would be no better than the Archaea fixture.

So, in my opinion, you can grow the plants you mentioned, with no problem, using just that Oddysea fixture, but with bulbs of no more than 10,000K color temperature. You might even need pressurized CO2 to avoid algae problems.

If I may, what Hoppy's suggesting is that with better bulbs than the 12,000Ks you have, you might even need pressurized CO2 so as to support the amount of plant growth your present fixture would foster so as to be able to successfully "out-compete" unwanted algae for the nutrients and light in the tank. I’d say that this would bring you outside of “low tech” territory.

I'd add that, at least from my experience, once you do add the CO2 you're going to need to support that growth potential with fertilization.

Well, then, I'd venture to say that unless you have the kind of experience that Hoppy has, if you do get the better bulbs, you might initially run into some problems. Once you add CO2, it isn't a simple matter to arrive at a balance of the three major elements of: light, CO2 and ferts when your tank isn’t heavily planted with fast growing plants and I’d say that it’s more than likely that you’d have some run-ins with algae and consequent poor growth of your plants until you can balance the tank correctly. I'd also say that you'd be quite limited in the kinds of plants that you'd be able to grow with just the two-bulbs.

But, at the same time, Hoppy's experience would tell you that it is possible to get it right and have a successful planted tank with the bulb change he suggests.