I can speak from some experience on the 770. Bought one earlier this year. Yeah I heard all the horror stories, but have seen a few at ranges that worked just fine. Wanted an extra 30-06, but did not want to spend a lot of money. Gander Mountain had the rifle package with the factory mounted Bushnell 3-9x40 on sale after Christmas that combined with their extra gun coupon savings brought the price to $199. Added in a padded sling and buttstock shell carrier for $20 (they give you a discount on excessories too if purchased at same time). So new rifle, scope, sling, and shell carrier was under $240 out the door.
Used for $400? NOT A GOOD DEAL!
Recent ads from BPS, Cabelas have the packages new for $259 all the time. Used value I would place on the rifle and scope set up you described is around $175 assuming it shoots straight and holds zero. You can pick up a reasonably solid knock-off of the Harris non-leveling bipod through Amazon for under $25 shipped so that doesn't add a ton of value.
If your budget is $400, there are better choices with smoother actions and more attractive lines. Would note that pretty much every new hunting rifle out the box made these days is capable of 1-1/2 MOA or less regardless of price, which is way better than most people can shoot in field conditions away from a bench. So accuracy is pretty much going to be equal for all the budget guns. What will truly be different is quality of build, using metal instead of plastic parts which should be more durable over a lifetime of use. You won't find a new rifle with good wood in this price range, but there will be plenty of aftermarket accessories and stocks available as well for other makes so you could improve them in the future.
So how is the 770? For its purpose as an inexpensive entry level hunting rifle the 770 is pretty good. Use it as it comes, I did not buy for the purpose of upgrading. Disassembled, stripped everything clean, reassembled, torqued everything, and locktited the 1-piece base (the front base screws were loose from the factory). It's about 1/2 pound heavier than my 700ADL. The bolt action though not smooth is certainly usable for hunting. Trigger has a little creap and overall is ok once you clean the gunk out. The scope is definitely not high quality glass. It has held zero over a few trips to the range and about 120 rounds. Prints right about 1 moa with my standard 180 grain hunting loads.
As a loaner that will likely be used by inexperienced hunters, it has a lot going for it. It is quite accurate and feels solid. I like having the detachable magazine so I can control the ammo. Thorns, brush, and rocks are not going to make me cringe. For myself, I feel confident it will work as a backup if something should happen to my regular rifle on a hunting trip. Overall a decent $200 gun.