Hi Stella,
Welcome to R2R and to this wonderful hobby. It sounds like you have put a lot of thought into your planning. I would agree with much of the advice you have received.
Whether to have sand or go bareback is a personal issue. I don’t like the look of a bareback tank. For me it doesn’t look anything like the ocean bottom. Aragonite or crushed coral is not expensive. It will turn “live “ on its own.
The Chinese black ones like Marsaqua or Reef Radiance are relatively inexpensive and with good maintainence can last. You can grow coral with them. You will need at least three 165 watt units for a 125.
I personally would use a refractometer for salinity. I would also use Hanna checkers for other measurements. You are a long way if ever from needing probes.
The eheim compact pump is OK but I have had one fail on me. For my money I would use the Eheim 1262. It has been around a long time and is very reliable.
If you are going to quarrentine fish, do it for a least three weeks. This is the minimum period for the marine I’ve life cycle. Seven days is just observation not quarrintine.
Definitely get circulation pumps. Either two to three Jewels or one to two gyres.
Don’t use the python. I just use a length of rigid PVC pipe and 1 inch flexible tubing you can get at Home Depot for $10.00. I use two 32 G Brute garbage cans on castors. I only use two. I don’t store ROI water. I just mix up as needed.
I would start out with both beginners corals and some hardy fish. It will make the tank much more interesting. You want to try to achieve a balance. Corals don’t just live off light. They also use dissolved inorganic in the water. Most of the nutritionally beneficial dissolved organic come from fish poop.
I may have missed it but I didn’t see a Protein skimmer in your plans. If you are going to have a reef tank a good skimmer is essential unless you have a very large refugium.
You have done the most important part by researching and finding good advice
I have had a reef tank for 37 years and I am still learning
Good luck