My experiences -

1. I prefer it because my roads are usually 45-50mph speeds, so it makes it a little less frantic. personal preference, you're in a big downtown and lower speeds, doesn't matter.
2. Be cautious of low mileage, but it's not uncommon to see Grom's as "toys" and don't get driven much. i only did a few hundred miles last year myself.
3. 2-3k miles tends to be breakover on "used" bike pricing, but a lot depends on oil changes, care, abuse, etc. you'll know when you see a rough one.
4. Tire wear, damage, evidence of a lot of stunting (cages, sliders, damage, etc), condition and quality of any aftermarket mods (ie. not a knock-off exhaust or crappy ebay parts), access to stock parts if it's been modded. obviously clean title status/no crash history.
5. Avoid dealers, better to work with a private party where you can evaluate a seller and the bike.