IMHO, don't waste time with the relay board. You don't need it. It's just something extra to buy and it looks like a science fair project.
Do invest in the wiring harness kit.
https://www.diyautotune.com/catalog/1239-megasquirt-wiring-harness-ms1-ms2-ms3-ready-p-43.html
I would gladly pay $90 bucks for all the time I spent building my own harness only to be unhappy with my work. And now all the wire I bought at the autoparts store is starting to rot.
The fuel pump situation can be a pain, but after several attempts I came up with a situation that works well for me. I have a small pump that feeds a large filter. The large filter acts as a swirl pot for the high pressure efi pump. Without this, my car would starve for fuel every time I pulled a hard corner with less than half a tank.
Do invest in a wide band O2 sensor. I use this one:
https://www.diyautotune.com/catalog/1239-megasquirt-wiring-harness-ms1-ms2-ms3-ready-p-43.html
Just be sure to never ever start the car with the O2 sensor unconnected to the harness. Your sensor will be immediately fried. And new senors are expensive. I am on my third. D'oh!
Get an old beater laptop with a serial port on it. I had one, until I sat on it. It worked great up until then. Now I use my "day to day" laptop with a usb-serial adapter. The usb-serial adapter causes all kinds of problems.
If you really want to build the Megasquirt itself for the fun of it, go for it. But if you are only buying the kit to save money... don't do it. You will be glad you bought the assembled unit. The project offers plenty of soldering opportunities without assembling the kit I assure you. I saw a pretty good deal on a MicroSquirt unit on eBay the other day.
Oh... also make plans to control your spark with the MS as well. Of all the mods I have made to my car, that has yielded the most performance gain (according to the butt-dyno).