> when 7700K reaches 5GHz in OC
To overclock it and not to burn your motherboard at the same time you need a motherboard with proper VRM, while in case with the 8400 any cheap motherboard is enough. Moreover, 7700K without delidding does not reach 5 GHz, delidding requires $$$.
> it's not stupid to go for 1st gen Ryzen
No it's not stupid and it's also not really smart. The 2nd gen Ryzen might support the max frequency of up to 4.2-4.3 GHz while the 1st gen only supports 3.8-3.9 and rarely supports more than this. Moreover, new motherboards might fix issues with RAM because now you need Samsung chips and nothing else to get your RAM working at frequencies higher than 2933-3066 MHz which pretty much suxx, plus there are troubles with ASUS motherboards when it comes to 2933+ MHz RAM at all, no matter if it's Samsung or not. Taking into account that Infinity Fabric is linked to RAM frequency, you want to have as high RAM frequency as you can.