1. would this M2 Mini be a lot faster than my 2015 iMac? my iMac is 3.1Ghz where this mini is just 1Ghz.
Your iMac has an Intel processor and the M2 mini has an Apple Silicon processor. Just about any Apple Silicon proceesor, at this point in time, will be significantly faster than any Intel processor.
The clock speed (Ghz) can only be used as a measure of relative performance between two otherwise identical processors.
Here is a table of how various models of Mac compare to each other performance-wise:
https://browser.geekbench.com/mac-benchmarks
You can see from the table that an M2-based mini offers more than twice the performance of a 2015 iMac.
If your 2015 iMac has a rotating disk hard drive, rather than an SSD, and especially if you are running Mojave (macOS 10.14) or later on your iMac (which means that the RDHD has been non-optionally re-formatted as APFS), your 2015 iMac will seem like a slug compared to any Apple Silicon-based Mac.
2. never bought a refurb before. do they come with the standard 1 year apple warranty? are they repaired by apple?
If you purchase an Apple refurb directly from Apple, from their online store:
Save up to 15% on a refurbished Mac. Tested and certified by Apple including a 1-year warranty. Free delivery and returns.
www.apple.com
...it will be indistinguishable from a brand-new Mac. Back when Apple still made Macs here in nearby to me Elk Grove, I toured their factory and found that refurbs are brand-new Macs that come down the assmebly line and they don't pass a test. They are then taken off the assembly like, and repaired and tested several times by hand. To you and I, this would simply be a new Mac. But apparently legally Apple can't brand them as brand-new. Most folks consider them better than new because they have been quality tested several more times than a brand-new Mac. A genuine Apple refurb comes with a standard one year warranty, just like any other Mac.
HOWEVER, if you purchase a refurb from an Apple brick & mortar store, you don't know what you might be getting. It might be a return that someone simply changed their mind on, or it might be a return that was a lemon. It will probably have been repaired in-store. So purchasing a refurb online is a safer way to go.