Welcome to the group! I know you'll enjoy the bike once you get all of the initial quirks worked out of it. I have a question about your carb leak: is it wet, or is it dry? Namely, is it still leaking, or is it just a stain from a previous leak?
I would run some Sea Foam through it. If it's something little, like a bit of crud under a seat or something, that will clear it out and save you a rebuild. If you take the tank off, which isn't tough, you can flush it yourself for about $5, if that. But, I don't think rust in the tank is the problem. This model has a full inline fuel filter, so I doubt any rust shale could make it to the carb and cause a leak. But, while you have the tank off, you could rebuild the carbs if you felt so inclined. Search this forum for the removal procedure. It's a bit tricky since the air box is integral to the frame and non-removable. I got my two carb kits for $55 for both. A few hours time, a can of carb cleaner, and they're back business. But if they were rebuilt 3 years ago, it sounds like they just need some Sea Foam.
As for the temp gauge: what was the outside air temperature when you were riding? On mine (which is working as designed, and others will vouch for this behavior as well) the needle never comes up past C unless it's 80 degrees out or more. Even then, it only comes up past 1/4 if you're stuck is traffic. My ride in this morning was 49 degrees, and across the entire 42 mile trip, the needle never moved. Not even a little. Unlike car, this bike has not only a radiator, but also cooling fins. Unless it's really hot out, the cooling fins are doing most of the work, and the radiator is just there for looks. Only when you're stuck in traffic at 90 degrees does the radiator and coolant come up to bat. Then the temp gauge will tell you how you're doing.
Start the bike, and let it idle for 20 minutes. If the temp gauge doesn't move then, then you know it's broken. it will also be a good test to see if your fan works. It should click on just before the needle get to the red.
Currently Running Stable:
2010 Honda NT700V
1986 Shadow 700
1986 Honda Trail 110 (Postie Bike)
1987 Honda Rebel 450
1973 Honda Mini Trail 50