"Are there any concerns? E.g., certain minimum distance that should be maintained between the HVAC and water heater or between the water heater and wall?"

You will find answers to all those question with installation instructions included with heater or advertising for heaters under consideration.

You need to leave sufficient space to service your HVAC unit.

Why do you think you a 75 gallon hot water heater? How many consecutive showers and length of each are you trying to supply each day?

Is this a gas or electric water heater?

If it is electric, consider a hybrid electric water heater. They come in the larger sizes and are the least expensive to run of most any water heater.

Yes, there are issues. You have to have an overflow pan under the water heater, plus cutoffs, and a drip leg, to bring it up to code. Plus firewall separation between the garage and house.

A complication you may encounter is water inlet size requirements for a 75 gallon water heater and may require increasing water inlet pipe size from 3/4" to 1".

I will raise the question if you really need a 75 gallon water heater or not. I'm assuming you have a gas water heater, and if your present 50 gallon water heater only has a 40K BTU input it's recovery rate will be low and so will the gallons of hot water in the first hour it will supply. Here are some examples of what you get from a hot water heater.

A 50 gallon water heater with a 40K Btu in put will provide 80 gallons of hot water in the first hour. Recovery will be slow.

A 50 gallon water heater with a 50K but input will provide 92 gallons of hot water in the first hour. Recovery will be faster.

A 50 gallon water heater with a 65K Btu input will provide 110 gallons of hot water in the first hour. Recovery will be the fastest.

A 75 gallon hot water heater with a 75 Btu input will provide 150 gallons of hot water in the first hour. Recovery will be fast but not as fast as the 50 gal. 65K Btu input.

It looks like a no brainier but in reality the 50 gallon heater with the 65 BTU will recover faster than the 75 gallon heater because there is 33% less water to heat up. So do you need an additional 70 gallons of water over the first hour if your present BTU input is 40K? Or is there time between usage where the recovery rate of a 50 gallon 65 BTU input would benefit your needs better.

In summary look at the BTU Input of the units and not just the tank size.

If the hot water heater is old and you have hard water, then it may not be producing the rated amount of hot water. A new hot water heater of the same size and a higher BTU input of the same size will produce more hot water.

" If you are already using a heat pump for your HVAC unit, this could be a really good opportunity to consider integrating a heat pump into your hot water system. "

Now that's an interesting idea. Several of us are already using a heat pump for our hvac system. Will you please expand on details of integrating our existing heat pump into our hot water system?

I'm actually confused by other things you said but maybe your explanation will clear those up.

michaelkaiser1965 "If you are already using a heat pump for your HVAC unit, this could be a really good opportunity to consider integrating a heat pump into your hot water system."

"With the right planning, you can not only expand your hot water capacity but also increase the efficiency of your entire heating and hot water system."

I too am confused by these statements. My heat pump and hot water heater have nothing to do with one another. One could replace a conventional electric tank water heater with a hybrid model -- they make noise and cool the room -- but what that has to do with ones HVAC system is a mystery.

A potential connection between HVAC and water heating is a heat recovery loop (I believe called a desuperheater) on a geothermal heat pump. My neighbor has such. He often turns his electric water heater off and gets sufficient heated water via summer cooling. Winter heating probably not so much as it'd be counterproductive to heating the house.