Since you have gas hot water heater, and obviously have not done one before, it would be better to call a plumber, so you know it is all done to code.

If your question is whether the lines are deteriorated too bad to reuse the answer is no, they are still serviceable. Just drop back to threaded fittings and add new lines going to heater. You can replumb water and gas using threaded adapters and flexible lines,

On gas line I suggest uncoupling union joint below shutoff and discarding nipple from tee to heater t-state. Inspect drip leg for collection of tar and clean if needed. Use 1/2" male pipe thread to flare adapters at tee and new t-state then connect with flex gas line.

Is not a Permit required in your jurisdiction?

That flex duct is a giant scary NOPE. All ducting should be RIDGID ducting. There's already too much done wrong in the picture, and that means there's probably no strapping, or pan, or etc. Call a pro and get all that straightened out.

well - looks like i gotta call a "pro" and cough up 3k in labor.

$3k labor !!!!! If you lived next door I'd furnish supplies and tools plus teach you how for half that. What part of the world are you located and how much does it cost to have a tooth crowned?

haha!!!! nooo i was just exaggerating. I got a quote for 2,100 parts /labor. i am in northern VA so ill see, maybe get some more quotes.

I just replaced my 14 year old Hybrid Hot Water heater with a new Hybrid Hot Water Heater - same capacity, same footprint.

The Rheem 50 gallon capacity unit was $1500+ The Installation by a Home Depot Contractor was $2504. (I am now too old and weak to do the install myself.)

I have a dentist appointment I'll find out what that costs soon enough. I love snide comments from folks that rarely actually contribute any added value

yes sir better not to do it yourself. Just call a pro.