Easy there, you're going in a few different directions and not all of them are good.
First off, today's modern ammo really negates the advantages a .45ACP used to have over a 9mm, to the point where the big 3 (9mm, .40S&W, .45ACP) all deliver about the same performance. This isn't 1940. Obviously 9mm rounds are smaller and lighter, which means you can carry more all else being equal. This is a big plus. It's also one of the most prolific calibers in the world, which is another big plus.
Second, shooting .40S&W ammo out of a 10mm is kind of silly. Why you'd want to carry conversion parts (?) is a little impractical. Autos don't work like revolvers in that you can shoot multiple loads out of the same gun without modification. Many .40 pistols can be converted to 9mm with a a few parts (barrel, mag, etc.) if you're really concerned about it.
Lastly, if price is any kind of concern, get the best pistol you can afford in a caliber you can afford to shoot the most of. A $400 glock (if that works for you) in 9mm and a couple thousand rounds is far better than a crappy .45 with 250 rounds. I'm plenty happy with my M&P9 and 17 rounds.