The answer to this question is it depends. If you are running Loki, the kernel isn't all that old.
Provided you aren't using anything really exotic, Linux typically supports most desktop hardware. I can't remember the last time I had an issue running Linux on a desktop machine - normally issues arise when trying to get it to run on a laptop, where there is poorer support for some chipsets (wireless etc.).
A better way to do this is to use something like PCPartPicker to assemble a potential build which suits your needs, and then post to the elementary OS subreddit to ask whether your hardware is supported.
Here are some things you might want to avoid:
- 32-bit UEFI firmware
- Older AMD graphics cards