"Must it be an i7 processor, or is an i3 good enough"

Most, not all, but most, do not use multiple cores but the faster the CPU is better. So, sometimes an i3 can be faster than an i7 when the clock speed on the i3 is higher. However, if you can afford either one without issue, i7 for sure as you never know when things will improve in lower-end CAD kernals. Most higher end CAD type tools cannot use more than one core. Some design tools though can and lots of render engines will see and use multiple cpu cores.

So, if you have the coin for the i7, go that route for even if you do not utilize the CPU today there might be some tool that you get in a year that totally kicks it on an i7.

nvidia card for sure, just make sure it is CUDA.