Yes, that sounds like a plan. My thoughts:
- I would top off the coolant and keep it topped off while you work on the other issues. The flush and fill is a low priority. It's more of something to do to be safe because you don't know for certain what kind of coolant is in the bike right now. If you are lucky, it's all the original H-D/Peak coolant. I'm just saying that if it isn't and some mechanic tops off the coolant bottle down the road it could create problems.
- I would only get one relay and replace the one for the starter. Some of them seem to be good. When you switched them around it suddenly would crank. There was probably something else that wasn't working, but that wasn't a problem since the bike still wasn't running.
- The brand of filter isn't important. Be sure to check the oil for signs of coolant leaking into it.
- You want to see if the plug gets a spark, if it properly gapped, and the general condition of the plug. Here's a chart to troubleshoot the spark plug. There are others like it online.
- Here are some oil pressure sender videos, but they are all about other motorcycles and cars. I'll try and look up the details for the Street 750 in the shop manual when I get home.