Gas HW Heater Code Advice

Need some help from someone who is familiar with Michigan (IC) code. I'm going to need my 40 gal nat gas HW heater replaced and plan on doing it myself. I've done this a few times before, but never above the board where I acutally planned on pulling a permit and having it inspected. I'm not quite clear and what I will need to do to make everything legal, but I'd like to have it right the first time.

I'm also getting a new furnace, the estimator advised me that one thing I'd need to do is to increase the vent size from 3" to 4". Simple enough. What is there now is single wall. My understanding is that as long as I am more than 6" away from combustibles, single wall vent duct is fine, true?

Also, I see a line regarding using a pan if "damage is possible"; that seems pretty gray to me. Enough leaking water will damage anything won't it?! My tank sits on a concrete floor, near a drain, but also near the furnace and the backside of an interior wall. Does this dictate requiring a pan?

The new gas control and the old gas control surely won't align. Is using a flexible connection (12-18" or so) OK? The old one is all black pipe. Know I need to leave a drip leg.

Any other gotchas that I should watch for?

Thanks,
JDH

I am not familiar with your MI code; others will help. But I will offer a few comments for starters:
1) Certain things...like the flue...are primarily governed by the National Fuel Gas Code...which virtually everyone adopts. The water heater manufacturer will specify the flue requirements, and that spec is invoked by code language. Water heaters up to at least 40,000 BTU can normally use a 3" flue. If your heater is bigger, or if there are local restrictions, you might need the 4"
2) Any building code will spec 6" to combustible for single wall flue pipe.
3) Your comments about the pan are accurate. Most codes only REQUIRE a pan indoors, but again local interpretations apply.
4) Gas is a local thing. Here ( S. Calif.) we must use flex connections on gas and water, no hard pipe. And no one really gives a hoot about the drip leg. The water heater manufacturer probably recommends it, though.

Thanks for the input, appreciate it.