With impact wrenches you probably want to size it with whatever sockets he will use. I personally would prefer a 3/8" drive square on a smaller-sized impact wrench because that matches up better with the size of bolt and sockets I would use it for. However, if your husband only has 1/2" drive impact sockets because he uses them with a big air gun - best to get a 1/2" wrench even in the compact size. Or just buy a socket set too.

Those medium and compact impact wrenches all come in both 1/2" and 3/8" size - for the most part there's no difference in their power.
The little red one is quite a powerful unit for the size. Milwaukee's M12 line has some very unique and useful tools. Because they are 12v tools, there's certain cordless tools they don't make - like a big impact wrench (I think the one I mentioned is the most powerful in that lineup), outdoor power equipment (string trimmers, mowers, blowers etc.), big stationary tools like mitre saws, etc. However, because they are compact they do make a bunch of things not usually available in the 18v or 20v tools - such as staplers, one-handed reciprocating saws, rotary tools (i.e. a "dremel"), etc.
One of my favorites is the M12 ratchet:
It is super useful for automotive work. Also available in 1/4, 3/8 and 1/2" drive sizes, plus extended reach, etc. This is not an impact tool, so it's not for really tight or stuck fasteners - but it can speed up some jobs immensely.
Yes the batteries can last a long time with infrequent use. I don't use mine daily - maybe weekly. Some of mine are about 8 years old now and still going strong. Ridgid, by the way, will also warranty batteries for life.
I prefer to have at least two batteries so I can swap if one dies, but that depends a bit on what size you get too. I.e. some special buy kits will come with batteries as small as 1.3ah - that's tiny! Regular batteries start at 2.0ah.
I'd suggest two batteries unless you get one that's 5ah or larger. Still best to have two, but at that size you're going to get lots of runtime. They don't have to be the same. I.e. one 5.0ah and one 2.0 might even be an advantage so you can use the smaller and lighter battery when the runtime isn't needed.