A lot of times, lower range hubs with sealed bearings are just nasty feeling at first and will [ahem] "smooth out" after some miles (as in wear) is put on them. As for lack of lube... Shimano is also pretty notorious for not lubing some of their components nearly enough.
As for the "sealed" bearings... are you sure? There is also a difference between "cartridge" bearings and "sealed" bearings (like Phil woods). Cartridge bearings are indeed set into their own casings - making them super easy to just snap in place on a hub or whatever during factory assembly, but in fact are sometimes serviceable and just removing the simple plastic closure caps may allow you to access the bearings to pump some additional grease into them.
One example which comes to mind are Shimano Dura-Ace (HP-7410) threaded headset bearings - which everyone assumed were permanently sealed - not so. They are actually even described in the Shimano service manuals as "Cartridge Type" bearings (and which actually contain 18 x 5/32" balls within a maze of shields to keep out water and grit). So, when a customer had lamented about his headset being gritty, too tight, and just shot and in need of replacement, we just popped them open, lubed em up, and Bingo! Perfect. That'll be just 5 bucks, please. Yeah, he was shocked.
NOT advocating that you bust apart a set of brand new hubs, but just a thought you may want to bounce of the LBS mechanics to see what they have to say...
Measure the spacing between the locknuts. As others have mentioned, most modern/current 27" replacement wheels are spaced 126 mm. And, often they have an extra simple spacer set on the non-drive side. This gives lots of room on each side of a hub and minimal wheel dish to make for stronger (and easier) wheel truing which will hold up better for the average hands-off consumer. But you can usually re-locate a spacer onto the drive side to fit a wider (7-speed) freewheel. You'd then probably have to redish the wheel just a bit though, to keep it centered.
Since your freewheel cogs are probably going to be... uh... 14-28 or 14-28... same-same whether using a 6-speed or 7-speed freewheel. I'd just use what fits and make it easy on yourself. If buying a new freewheel, I've gotten some pretty nickel-plated Chinese-made House Brand 6-speed freewheels from Nashbar, on sale for only $12 and they even have the nice modern HG tooth profiling. They seem to be holding up just fine on a beater/commuter bike. And a couple friends are still abusing the same on their beater bikes too. [BTW: Did you know Shimano has all of their modern replacement freewheels made in China now too?]
Enjoy!