"What calculations did you use to get 1000 years anyway?"
I'd always heard 100-150 years for an Orion craft to get to a Alpha Centauri, a lot of those calculations don't include the energy required to slow down for some reason (I think) so I multiplied the estimate by two (200-300 years). Alpha Centauri is about 4.3 ly from us so I divided 16 by 4.3 and multiplied that (3.7) by the 200-300 estimate to come up with 744-1116 years. Of course all of these numbers are probably based on the original 1960s Orion so with modern tech it might very well be possible to bring the time down quite a ways. And with research into more advanced propulsion it might be possible to bring it down even more (spiked fusion, antimatter, Bussard ramjet). But a major design consideration no matter what the technology (unless its some FTL tech) is going to be time. Any materials/technology that you have on board is going to have to be repairable, manufacture-able and recyclable on board. Sure you could make some ships components out of carbon fiber, but you wouldn't be able to replace them so your probably going to use aluminum. Sure you could use top of the line computer processors, but if they fried you'd have to have replacements (and hope age hadn't killed them) so you'll probably go with an older design that can be built on board. Any of these would of course require a massive ship be built, but even that isn't all that improbable, for the likely final cost of SLS alone we could launch the mass of a WWII aircraft carrier into orbit on today's commercial launchers. Times that by 10 and you've reached the mass of a moderate sized Orion (or about the initial cost of the Iraq War).