gamesfreak64: So the bad news is the average hardware ( that is affordable to most of the people) doesnt work or will have many problems trying so ?
The affordable H310/B360 Cannon Lake chipset boards won't work with W7. The USB doesn't work at all. If you want a Coffee Lake CPU and Windows 7, then you definitely need a Z370 (or B365) board (Kaby Lake refresh chipset). It doesn't matter what CPU you put into it (G5400, i3-8100, i5-8400, i7-8700K, etc) or what brand of board (MSI vs Gigabyte) but it does definitely need to be B365 / Z370 and not H310 or B360, so yes that lowers the affordability of new budget Intel rigs with W7 compatibility.
Edit: There are new versions of the H310 called "H310 R2.0" that are W7 compatible but they're not always clearly marked.
gamesfreak64: What about this hardware: will it work ? and once done what would happen in case you have a dual boot? win 7 / win 10
MSI Z370 PC PRO
Intel Core i3 8100
Corsair DDR4 Vengeance LPX 2x4GB 2400
Corsair DDR4 Vengeance LPX 2x4GB 2400
Gigabyte Z370P D3
Intel Core i3 8100
Z370 + i3-8100 will work on W7 though it'll require a dGPU as Intel refused to release any driver for its UHD630 iGPU. Your only other cost effect alternatives are:-
1. A Ryzen based system, eg, B350 + R3 1200 or 1300X (but not the 2200G / 2400G APU's), or
2. A cheap H110/B150/B250 Kaby Lake motherboard and then find a 2nd hand i5-7500 on EBay (same 4C/4T @ 3.6GHz chip as i3-8100 at same price 2nd hand). This doesn't give any upgrade potential to hex-cores (which also isn't guaranteed for future 2019-2020 Ryzen chips running on W7 either) but there have been some bargains recently if 4C/4T is all you need and you don't care about future heavy-weight multi-player games.
Dual-booting between W7 and W10 is also no problem. I've tested that myself. If you install W7 first then W10 after, you get the W10 GUI boot-loader, if you do the opposite you get the W7 text-based boot-loader. Both work fine though either way. W7 does need the USB drivers "slip-streamed" though as mentioned earlier but that's a simple thing to do that takes less than 10mins.