I'd actually heard the same, that Angel was always intended as a love interest for Buffy but his actual role and story wasn't decided upon until the role had been cast and the beginnings of season one already filmed.
Obviously, it would have changed things substantially. Angel's entire core story would have been fundamentally altered. His principal storyline through the first three seasons, a vampire with a soul, wouldn't have existed. I think that Angel always would have been Buffy's primary romantic, but things sure would have been different. Angel would probably have been a distant, mysterious hero, and vampires would have probably still killed his family, but it clearly would have been other vampires. However, I think that aspect would still have originally served as his driving motivation (along with Buffy of course). As to whether or not Angel still would have landed his own spinoff? Not sure, there's no real way to tell.
Personally, there's not much, if anything, I would change about Angel, and certainly not anything important. I think his story, on Buffy (always love Angel on Buffy 😀) and continuing (and evolving) on his own series, is by far the most compelling and interesting. His relationships with other characters (particularly Buffy, Spike, Cordy and Wes) have so many layers to them. Every season of AtS drew, and still draws, me in, so much. I just think he's a phenomenal character who always has a great story to tell. And his S1 story with Buffy? Call me a sap (which I am), but the romantic in me just loves it. The episode "Angel" is one of my all time favorites.