Standard 280 ammo should shoot in an improved chamber. If this is your first custom and I know your fairly new to reloading, I'd stick to the reg 280 Rem. Then at some point down the road if you really want the improved version, it's a simple reaming job.
If I remember right improved chambers are cut a bit short, .006" if I remember. That is done so that the reg factory case will be able to be fired in the larger chamber. The improved case has the shoulder changed and the factory round won't head-space properly without that short chamber. On chamber's cut to length, hand-loads are performed because there will be excessive head-space. Then the bullet in the fire forming loads is loaded just into the lands to hold the case where it belongs. Without that you risk getting case stretch in the head as the firing pin drives the case forward and when fired the pressure pushes the case back after the neck and shoulder area has already gotten hold of the case. The 280 AI will have almost no body taper and a sharper shoulder. It's advantage over the standard 280 just isn't that great. Not enough to justify it. The reg 280 is right on the heels of the 7mm Rem Mag with good hand-loads! Factory 280 loads were a bit anemic because of the actions they were introduced in.
I reload a lot but if I didn't, I think I'd take a good look at the 260 Rem and the 6.5x55. The 6.5x55 is a much better cartridge when reloaded and the 260 might require it in the future sometime. But both are easy to load and the 6.5x55 is very accurate. I've never shot the 260 but that should be a really fine cartridge. It's not much more than a 308 case necked down. In wildcat form it was called the 6.5x308 and the 264 express.
Someone mentioned the 25-06 and for antelope and deer it has no flies on it. I shoot the 117gr Hornady's in mine but have also used the 100 gr Hornady's. It's very accurate with either bullet and will leave the 243/6mm in it's dust. Hunting weight bullets from 100 to 120gr and speciality bullets even heavier from wildcat bullets. I have shoot a lot of deer with the 25-06 and a few antelope and with the 100gr bullet they simply fall over with a good bullet placement. With the 117gr bullets they often go about 20 yds and fall down. But a bad placement with the 100gr bullet is prone to ruin a lot of meat.
I'm surprised more people haven't mentioned the 270. It is a good cartridge. I've had one but never hunted with it, just liked the 280 better but what one will do, so will the other.