So you are predominantly interested in the pricing thing? Uh, alright:
- The things you said you plan to buy are kinda' fine for the 625 dollars. Where I live, one can expect 650-1000 dollars price for this, talking in general. But it isn't only price, if this is used, the seller should give you s.th. like benchmarks or else to make you sure all is working fine.
- i5-8400 + 1070 Ti + 16 GB + your mediocre power supply + 1 TB HDD = 360-550 dollars for these 5 things
Anyway, I still wonder wtf is going on here.
- Your 1070 Ti is hitting 80 degrees. Did you try putting thermal paste MX-4 and some custom cooling, like removing them weak fabric fans and putting 2-3 120-mm case/molex/cpu header/etc fans instead? The 2060 'll have lower temp, but do you think this is the way to deal with the problem?
- Stutters in performance...as you said, more threads may help and the i7 to do the job. The i7 here seems a fine idea, but I am not sure the 1070 Ti is to be blamed. Also, I am not sure if you tried vsync and limiting the FPS, not to mention undervolting the GPU to reduce the stress onto it, at least for test.
- Your motherboard died. Can't you put another mobo and keep it alive? I guess ASUS 'd be more durable.
- Both your current and your potential PSU are mediocre in the terms of quality. It's not impossible that your own PSU has 'helped' damaging your dead motherboard.
- You have to do some benchmarks of the things you sell, memtest64/Windows memory diagnostic, HDD scanning software tests, etc, otherwise it's sucky. Would it be games, benchmarking software such as Novabench or else, is your choice, but you'd better do it. What happens if you actually sell a defective item to s.o. else?
- As for the ram thing, your own stick 'd worsen performance in case it is lower frequency and higher timings (latencies). However, if this difference is low, the Dual-channel mode may not only neutralize this, but also give some improvement.
- Also I wonder why don't you have an SSD. Even a 10+k rpm HDD cannot outperform an usual sata SSD.
If you've tried overclocking
anything, I think this is the moment to realize overclocking is pretty useless. Even the fabric-OC GPU are things that I try to run away from.
The things you think are dying may actually survive, but if you be a careful user. The potato-laptop I write from has survived for 13-14 years. I did replace some things, but it was recently and nobody complained for the few older things I sold 2nd-hand from it.