It's not true that an i5 isn't good enough, Intel's naming scheme is all over the place but the difference between an i5 and i7 for this system is just that the i7 has hyperthreading and the i5 doesn't. It does give a small boost in heavily multithreaded workloads but it's not day and night difference as you can see here with encoding performance:
www.anandtech.com
Graphics cards can make a significant difference to encode speed and the GTX 1070 although a couple of generations old now is still a powerful graphics card. You'd need to check which software you're using to see if it does support GPU acceleration and how much it can assist.
I was using a similar system up until last year with an i7 3930K (hex core rather than quad core), GTX 1070 and 32GB ram which I found had good performance for 4K editing. I changed to a much newer 12 core processor which was faster per core as well as twice as many cores but didn't see as much improvement as I was expecting in 4K rendering performance because the graphics card was doing a lot more of the work than I'd realised.
If it's a friend who has the system could you load up your software and try a test render you've done on your laptop to see how the speed compares to give you an idea whether it's worth it.
I'm really not sure how to calculate the value as I've no idea of Canadian prices and in the UK, the prices for old Intel and Nvidia hardware can be surprisingly high. In addition to that the current PC market is a bit of a mess because both Nvidia and AMD have launched their new graphics cards but they're almost impossible to buy which has started pushing prices of some of the older cards back up. My main concern about buying an old system like that (the processor is six years old) is that there's no upgrade path, if you want a newer processor you'd also have to replace the motherboard and ram as well. If you have a failure it can be hard or uneconomical to source new parts and old second hand parts may not be that cheap and could also fail as well. In short I guess I'm saying I wouldn't spend a lot of money on it and you can check against current prices to see if it's worth it.