One handgun to do everything we want handguns to do?
Hmm. .45 or .357 are the choices I'd spend time on. I'd want a 3" barrel for .357, but I don't know if you can get a 3" .357 for $400. Hopping over to gunbroker, I see one available right now that I'd consider. I'm ruling out Rugers because I've never had a Ruger with a DA trigger I could tolerate. I understand that may be a "minority" opinion, but there it is. So there's one Taurus available right now, and it might go above $400 before the auction ends. So call the .357 a good idea, but difficult to fit within the price ceiling.
So let's look at .45. I've been shooting 1911s for decades. If I'm limited to one gun, it's going to be JMB's masterpiece. 4" 1911s are reliable, have the classic 1911 ergonomics, and are easier to conceal than the 5" original. I see a dozen metal framed (one gun? It ain't gonna be plastic) .45 Commanders under $400. I think my answer is RIA or ATI .45 1911 with the 4"/4.25" slide.