Don't get something that's less than 8th gen unless you're getting it for an amazing price.
The 8400 allows you to use cheaper motherboards, and honestly if you're using a 1080p60 monitor that's all you need.
If you do high refresh rate gaming, then I'd consider the 8600k.
Don't get an MSI motherboard. Asus, Gigabyte and ASRock are all better choices.
I don't think anyone will make fun of you for buying stuff on eBay. A good chunk of my entire build was done on eBay, and as a result I got a killer computer for $350.
There's nothing "sad" about it. That's just the reality of the matter. Software demands for the consumer market haven't been escalating to the point where hardware is getting left behind. There's a video on YouTube of the 8-bit Guy installing Windows 10 on a Core Duo Macbook from 2005, and it ran fine all things considered.
I mean, let's face it. Most people just use PC's as a telecommunications device. Go on the web, browse web pages, scroll down Facebook and Twitter, etc. etc. Type out a document in word, make a spreadsheet in Excel. That's not something that necessarily demands a ton of hardware grunt. The fact that machines like Ultrabooks and Chromebooks exist is the silent acknowledgement of this: devices that focus on reducing power draw as much as possible to try and maximize battery life to its fullest exists in a thin and light form factor. Even Intel's Core M was explicitly designed and marketed for this purpose.
Even in your example of the Macbook Pro vs the XPS, those machines are focused on much the same just only with more powerful hardware. And they're good machines, you can do things like edit 4k content and game on those. They cost a premium, but they're premium machines in the first place.