You need to do a little more archeological digging still. You need to know the whole construction of the floor before you can decide how best to install something new. I'd take up that plywood piece in the kitchen and see what's under it. It's higher than the surrounding floor, so needs to go. You should also be able to see from there what subfloor is under the original kitchen hardwood. It's too bad the space under your entryway is finished, but I'd still do some exploring from below to see what's under there, and to figure out the fix to stop it from squeaking. Is it drywall below or a drop ceiling? I know it probably seems like a pain to tear out drywall, but if you cut out a clean patch, like 2' x the width of the joists, it'll be relatively simple to repair. Just make sure to cut it down the center of the joists so that there's room to screw the patch piece to it later. Cut the hole under the area that squeaks the most, if you can. Have someone walk on and watch the floor from below to see what's moving. It might be as simple as pounding in a couple shims from below, or face-nailing a loose board from above. You want to find out what's underneath the hardwood. It might be laid directly on the floor joists, or there might be 10" wide boards that the hardwood is nailed to. Another thought is to take out a board in the hallway, since that's not original and you want to replace it. See what the subfloor is there. I am a bit puzzled at why, when you've stripped the kitchen down to the original hardwood, it's still higher than the hallway floor. Is your current surface perhaps not the original kitchen hardwood? Peeling back the plywood area will help determine that. BTW, I haven't heard of that stop squeaking product you linked to, so can't offer any personal opinion. And don't pour self-leveling compound over top of hardwood. It needs to go over plywood.

What a great house, congrats!! I've had old wood floors for the past few years in apartments I've rented. Sweeping often and cleaning with vinegar and water is best. Regular sweeping is key. For a quick wipe down I put a rag on my Swiffer and damp mop w/the solution. I don't use the Swiffer for anything else - don't like the chemicals. We will be pulling up old carpet in our new old house as well. We have lots of scrapping to do i think, but probably not sanding thankfully. There are gouges, but they don't bother me. Also, I saw a thread about Boda recently that I'm going to research more. I like to get a more thorough washing done every month or 3 then a damp mop can do, but don't want water sitting on the floor, and cant do the hands n knees cleaning anymore. Oh, also inspect your floor for gaps. From apt living where the floors haven't been taken care of I can tell you the gaps fill quite nicely with crud :P I don't know how to address that if it comes up though.

I've dealt with 30 year old engineered floors with the original finish, and a couple of screening and topcoating over it. If you take care of floors and don't let kids play roller hockey indoors, you too will never need to do a sand and refinish. The idea of "refinishing" as a criteria for choosing your floors is not really a valid concern. Choose a quality major brand, and you won't have any worries. No LL or box store crapola. $5-6 a square foot and up is where the good stuff starts. Under that, you can get some OK buys, but you're generally gonna have to do a lot more homework to make sure. So, what does "taking care of your floors" entail? Vacuuming a couple of times a week. Not allowing food and drink outside of the kitchen. Using rugs at all entries to capture the dirt from outside. Being a no shoes indoor household helps a lot too, but I'm not going to get into that classic arguement. Don't mop your floors. You can damp clean them, but people get all hung up over sloshing water on top of wood, and that's just a comple no no. Use something like a Bona cleaner a couple of times a month, max. Every other type of cleaning should be spot cleaning. But with food and drink confined to the appropriate spot, spills are minimized. Abrasion from debris is minimized by rugs that you frequently clean. And in 8-12 years, you do a refresher of a screening and recoating. You don't need to sand down to bare wood at all. Unless you don't do the above and wear through to bare wood.

When it comes to hardwood, trends are something to ignore! Why? Because hardwood is pretty much a lifetime purchase, and unless yours is damaged in some way, you're probably not going to replace it. So, light wood vs. dark wood, wide planks vs. narrow planks -- that stuff's all going to come and go. With that in mind -- as well as the very real concerns about cleaning dark wood -- I'll vote for a nice, neutral midtone every time. Not too much contrast, not too red, and (unless it's a beach house) not too pale. I think this is the wood that's most likely to give you good service and stand the test of time.