I suppose you're planning to spread the dropouts quite a bit?
I see you want to run an internal brake... It may behoove you to use rim brakes, under the circumstances....
That being said, the only IGHs that are rated for offroad use are the Rohloff (14 speed; costs well over a grand; will need to run rim brakes or find a way to mount a disc) and the NuVinci N360 (bicycle CVT, costs about $350; can run with rim brakes and can be retrofitted to accept a Shimano rollerbrake.) In both cases, you'll need to spread the rear dropouts about an inch to accept the hub, which might not sound like much, but from a metallurgical perspective it is, especially if you intend to ride offroad.
The IGH guys on bikeforums and mtbr seem to agree that Shimano Alfine 8speeds are bulletproof. Here's a pretty good price on one:
http://www.niagaracycle.com/categories/shimano-alfine-sg-s501-hub-kit-32h-black You'll face the same issue of having to spread the dropouts from 110 to 135mm. Like I said, Shimano warns riders not to run these offroad, but guys have been doing it anyway with good results, even with crazy low primary drive ratios. (Shimano, Rohloff and SRAM demand at least a 2:1 primary ratio; NuVinci suggests a minimum of 1.8:1...) Thing is, if you buy a new hub, you'll void your warrantee by rolling trails with it. Then again, I've never even tried to warrantee a hub, ever, so I don't know if that's really a factor.
You can find used/NOS Shimano Nexus 7 and 8 hubs in coaster and rollerbrake flavors; from online anecdotes, it seems like trail-riding tends to blow these up. Same thing with the 11speed Alfines, so far, but I suspect they'll improve them as time goes on. Shimano seems to do its IGH R&D by letting customers pay full price to test the wares....
From what I've read, SRAM's IGHs won't live long offroad. They have this new G9 which I haven't heard any feedback on yet, but given SRAM's track record of IGHs turning to guacamole on trails, i'd stay away for now....
This leaves the Sturmey-Archer X-RD8. These are spaced at 132mm, which is a small but noteworthy difference. These have drums; Sturmey doesn't offer a coaster version (but they do make a disc and a FW version...). This hub is weird b/c the lowest gear is also the primary drive, with 7 "overdrive" gears. There's a big jump between 1 and 2, then equally spaced 14% jumps between each gear from 2 thru 7. The 7 to 8 gear is another big jump, and even with a 1:1 direct drive, you'll be pushing about 85" in top gear. The good news is, since it's all OD gears, you can run crazy low ratios on these with far less worry about excessive torque eating up the hub's guts as much as it would with underdrive gears. Early versions of the Sturmey 8speeds were highly unreliable; internet rumours and hearsay suggest that they're much better now. But I'd still be a little pessimistic about the long term reliability. I will say that Sturmey-Archer's drum brakes are awesome, although prolonged braking will cause heat-induced fade, and a whole lot of it.
From a cost/performance/repairability/klunker-authenticity point of view, I'd say go with a 2x5 derailer set up. You'd save some weight, too, although weight weenies don't really build klunkers, do they? (The original guys building "hybrid" Schwinns were, however, very concerned with weight.) But, if you simply must have a 7+speed IGH, I'd suggest a Nexus 8 with the rollerbrake or, better yet, get some canti posts brazed on and run v-brakes with an Alfine 8.
Good luck!
**EDIT** Alfines don't come with Rollerbrakes, but Nexus 8 do....
