I'm currently using something I call the "6X47 Lapua" based on the 6.5X47L case. It's smaller and more efficient than the .243AI. It does what I tried to do with the .243AI. I shoot a 35 degree shoulder instead of the 40 because it still resizes well. It's expensive to set up though.
If you want K.I.S.S. simple then I'll suggest the 6BR.
IMO this should be everyone's first "real" custom rifle. It will establish a benchmark with no BS.
The two major problems with the .243AI are that you can't properly maintain the case (or at least I couldn't) and you can't run high enough pressures to really make it shoot to it's capacity. Furthermore, if you ever try to make a .243AI truly accurate, like competitive, then you have the problem of gapspace........ when you blow out an AI case it shortens quite a lot. This leaves a "gap" at the end of the neck that tends to fill up with crud.
Cool looking round, not a stellar performer. If I were dead set on using the .243 case I'd leave it stock, short-chamber it so that you can't get a factory case in without bumping the shoulder and shoot it at around 2750 with 105-108gr bullets. Done this way it can win. It doesn't beat a 6BR performance wise but it'll give it a run for its money.
Just remember this, when you're spending this kinda' money on a hotrod you don't need to be able to buy ammo at the local mart. And that's really the big selling point of the .243AI.
No matter what you choose, use Lapua brass..... or if you follow Haney's advice and get the 6MM use Winchester.
Try searching if you want to read a lot of opinions on the various choices.
al
This cartridge is a wildcat, no real SAAMI specs. You'll need to find someone who's done it before, or really research it and design or copy a reamer. I of course think that "my" iteration is the best, but others have theirs.