Here's an exhaustive list of what to check for when buying a G1 Insight, in addition to looking at the oil, checking the brake pads, and kicking the tires:
-Northern cars can get rust on the fuel and brake lines, especially right at the 90° turn at the bottom of the engine bay. This can be a big deal as I understand, but mine lived in the south and has new looking lines. Northern cars are also prone to the bolts on the catalytic converters rusting into a blob, but this is only an issue if you ever need to pull it off.
-They're prone to a very small roof leak above the passenger B pillar, but it's not a big deal. It won't get the carpet wet or mold, but often this will cause some corrosion in a wire behind the inner trim, and manifest as the rear hatch quiting locking and unlocking with either the FOB or electric door locks. This wiring problem can be fixed in 10 minutes with a toothbrush and some dielectric grease. You can see the fix in my build thread.
-All of the door lock actuators are kinda flimsy and prone to going out. Luckily, RockAuto has excellent replacements for <$25 and they're not hard to replace. You can still lock and unlock the doors with the key, but this issue will cause the FOB to not work. You can also see the fix for this in my build thread.
-3rd to 2nd gear downshift is prone to grinding, and moreso when the transmission is fully warmed up. It's not from from the synchros going out, but from a design flaw in the transmission. I haven't done it and only have a vague idea of why it happens, but it can be fixed by removing some tabs. Mine grinds if I don't double clutch in 3rd to 2nd downshift, or use push it slightly against first gear before dropping it into 2nd, and I live with it because double clutching isn't difficult and pulling the transmission is. Major bonus points for the car if it doesn't have this downshift grind.
-It's a 3 cylinder and vibrates a lot. Honda used soft engine mounts to mitigate this, but they don't last, and most Insights will need the rear mount replaced once in the life of the car. It's around a $90 part, but it's extremely accessible and easy to replace. This is detailed in my thread. Some people fix this by putting a bunch of caulk or other compound in the mount instead, but will suffer from greater vibration over the stock mount. If you get one and want a broken mount to play with, I'll send you the one I replaced.
-The EGR passages and valve will probably need to be cleaned once in the lifetime of the car. This can be done in about 20-30 minutes and costs nothing. If the car stutters and bucks when entering lean burn, this is probably your culprit. Mine didn't need it yet, but I cleaned it out anyway and there was definitely some crud building up. Details are in my build thread.
-An included grid charger can add value to the car, and indicate the owner was mechanically inclined and cared about the car. Most weak batteries gain back a lot of their lost capacity with a 36 hour balance on a grid charger. Aftermarket they cost about $200, but I built mine for ~$30 I think? If you're not handy with electronics, you might ask to borrow one from a member here or purchase your own. You can see details for how to build and install a grid charger in my build thread.
-The driver window switch has a cheap plastic piece in it that is prone to failure, and will cause the switch not to return to center automatically. To replace the switch with a factory piece it's around $200, but you can 3d print a replacement for about $3 if this afflicts your car. The links to what you need can be found in my build thread.
-The oil pan is made of magnesium and is easy to strip. A lot of Insight owners opt to install a Fumoto valve rather than risk some idiot at Jiffy Lube stripping out their absurdly expensive oil pan. Take a glance and make sure the stock plug is in place, or a Fumoto valve.
-If the rear engine mount deteriorates, increased movement of the engine can damage one of the many ground straps on the engine. I'm not sure how this manifests, but I found one strap on mine slightly frayed, and repaired it with some solder. You can also use any piece of wire for this, if you so choose.
-Check to see if all of the stock underbelly panels are in place. One of mine was damaged, and one was missing. These don't really have very much of an aero penalty if they're missing or damaged, but I made replacements anyway.
-It should be obvious, but check that the rear wheel skirts are not missing. They're expensive.
-Check to make sure the IMA light isn't on. This indicates a weak battery. A grid charge can restore it to some degree, but if the light was already on I'd probably pass on the car, unless it's a 2006 (and thus still under warranty).
-Check what tires are on the car. Bridgestone Ecopias or Dunlop Enasaves are great tires, but they trade a bit of gas mileage for improved ride and reduced road noise over the stock Potenza RE92's. I'd hassle the owner if the tires on the car weren't one of these three, as low rolling resistance tires make a TREMENDOUS difference in fuel economy. Stock size is 165/65r14.
-Check to see what the lifetime fuel economy is on the car. This can be found on the same display as the car's total miles. If it's below 55-60mpg on a 5MT, it was probably driven pretty aggressively. An Insight in good repair can get 70mpg with ZERO effort, and considerably more than that on individual trips. I regularly see trips to and from work in excess of 100mpg if the weather is good.
Aaand that's about it. These cars are not prone to any major mechanical problems, and even with everything above wrong, you can still get 70+mpg, but there are some little details that commonly crop up in a 15 year old car with 150,000+ miles.
EDIT: You might find this link extremely informative:
InsightCentral.net - Interactive Encyclopedia