You can't just swap out parts on a 1911. Well... You *may* be able to, but most parts require some degree of fitting and know how to install.
If you have a limited budget and want to shoot IPSC I'd go with something like a used M&P in 9mm. Don't misunderstand me - I love the 1911 platform, but it's not the best choice for saving money. Many 1911s require some amount of TLC to run flawlessly - and many folks don't have the patience to learn how to do this. The less you spend on a pistol upfront, the greater the likelihood of sub-par match performance. And having an issue during a match is sometimes enough to sour people to IPSC. If you have a malfunction at a match, you deal with it "on the clock".
Also, unless you are going with a 9mm, 45 ACP is horribly expensive to practice with (unless you're reloading).
I'd suggest going with something more inline with what you can afford - and doesn't require any modifications - to get going. Take the time to learn what actually works in a match and then get the 1911 that works for you under the conditions you need to work in.