New owner, old problems

Long post. Finally picked up a 99 k3500 w/ 454, crew cab, 8ft bed. Little bit of background I have almost a decade worth of experience professionally as a mechanic at my local county. The truck was sitting for a couple years due to brake line rusting out, and old owner picking up a new truck. I checked fluids (more on that later), jumped it and it started right up. (Slight tapping but seemed to quiet up when it got warm). Now that it’s at my house I’m running into a few things:

1. Put a new battery in, as old battery was toast. Also has a disconnect switch that I may remove. When starting with new battery it still acts like dead battery then it just cranks up. Possible high resistance in starter/ starter wiring.
2. I ordered a complete set of brake lines because if ones rusted out, the others aren’t far behind.
3. Coolant reservoir was empty, but radiator was full. After filling and running at my house, I noticed moisture/smoke coming from oil fill with cap off, common when engines sit for a bit it can collect a bit of moisture and burn off however; it looks like my oil dipstick is gaining fluid, so question where is a common coolant leak to oil on these? Head gaskets or lower manifold gaskets? I have the exhaust detector tool (blue liquid turns color if detects exhaust), have not used it on it yet but no oil is getting into coolant.
4. Gauges are erratic, fuel gauge pings to stopped on empty side, speedometer slowly creeps up to max speed when running. Oil, temp, and battery seem to read fine.

I just got this truck and am repairing it in my limited spare time with a newborn, so brakes lines are not on yet so it cannot be test driven in length. Any help or info to point me in the right direction would be greatly appreciated to help shorten the time diagnosing. Happy Father’s Day to all the fathers out there!