1911 mods, from what I've researched, are a whole new realm. Most parts need to be fitted to work 100%.
What are you wanting to do with the gun, exactly?
Personally, if I was looking for an all-out custom 1911, I'd go with one of the big name gun smiths and have them build it from the ground, up, to exactly what I wanted. In doing so, they'll most likely start with a bare frame and slide and go from there. Again though, this all depends on what you want to do with it.
If you're just looking for a 'nice' 1911, you could go the semi-custom route which start out at about $1,500 and go from there. Les Baer, Wilson Combat, NightHawk, etc...
You might even check out STI or SVI as well. STI has a bunch of 'custom' guns that they usually stock, and SVI will build you whatever the fuck you want. They've even got a neat little program on their site that allows you to 'build' your dream gun, and send it to them, and then they'll send you back a quote. Note, however, that both are rather expensive. IIRC, I priced out a full-on 2011 build through SVI and it came out to around $3,500 for a Stainless version, or $8,000 for a full titanium version. Yea. Won't be buying that anytime soon.