Maple is another great choice for hardwood flooring. Many people prefer its light tan look, which blends into home interior spaces. As one of the strongest woods, maple is a top choice for high-traffic areas of a home. The wood doesn’t scuff easily, making it a good choice for homes with children or those with a lot of foot traffic across the floors. Maple requires minimal maintenance, with weekly sweeping and annual applications of wood soap to prolong the life of the floors. Denver Hardwood Floor

First, most people are unable to successfully refinish the floors themselves. It takes a lot of experience to refinish properly. Most people can’t get it smooth/even and also don’t realize that you need to sand them 3 times before putting the stain on. It’s even harder to do it right if they are pine or maple floors (and if they are, they require conditioner before stain is applied). Also, once the floors have been sanded unevenly by a non-professonal, it’s even harder to correct due to the waves usually created by the non-professional. Denver Hardwood Floor

So, assuming you can’t refinish existing and you can’t afford to replace it now, then focus on the new kitchen and put solid oak there. It’s okay and very common for this to be higher than the rest and it’s easy enough to add a wood transition. And, then later, you will need to replace the existing worn down floors w/ solid oak, so they will eventually be the same height. Denver Hardwood Floor Install

No matter which type of flooring you choose, if the subfloor, joists or other structural elements need work, you can expect to pay considerably more in both labor and materials. Plywood for the subflooring averages $22 for each 4-foot by 8-foot sheet. For a 12-foot by 12-foot room, you’d need five sheets of plywood, which increases the cost by $110. Additionally, carpenters often charge $20 to $35 per hour to install new floor joists and subfloors.  For laying a hardwood floor over concrete, it's best to use a floating floor system where the floor is not glued to the concrete.  Wood flooring is a major investment, so always have the concrete checked for moisture prior to installation.
Eliminate any problem spots to make the subfloor level. Sand minor humps down with a hand-held or orbital sander to make the subfloor level. To fill any dips or low spots, use a leveling compound (also known as floor patch). Mix up the compound according to package directions, fill in the dips, then pull your straight piece of lumber back and forth over the spot to flatten and level it with the rest of the subfloor.[7] Hardwood Floor