To fix a buckled floorboard, you need to be able to access the floor from below. The way to tackle this problem is to put weight on the buckled area from above — a cement block works well. Then install a 1 1/4-inch screw in the buckled flooring from below. Allow the screw to penetrate only halfway into the flooring, or it may come up through the finished surface. Driving the screw through the subfloor and into the flooring pulls the flooring down against the subfloor and gets rid of the buckled spot.

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.
Installing a hardwood floor yourself can save you money – if you know what you’re doing. Improper preparation and installation can lead to warping and buckling, which will ruin flooring that would otherwise remain beautiful for years to come. The most important thing you can do to ensure the structural integrity of your floor is to properly prepare for the installation process. Here are various articles to help you!

Engineered wood flooring was created to lower the cost of wood flooring and attempt to fix some of solid wood’s shortcomings. It’s constructed from inexpensive plywood or particle board and topped with a thin veneer of hardwood. This construction helps with installation flexibility and slightly improves indent performance, but it adds a new set of problems as well. Because the “real” wood part of the floor is just a thin veneer, damage from scratches can be irreparable. Engineered wood floors are still made from porous wood, meaning they can’t be subjected to water. Denver Hardwood Floor
Glue down engineered hardwood, alternatively. If you don't want to nail down the boards, you do have the option of using adhesive to put engineered hardwood in place. It's a good choice, but won't last as long or be as sturdy as nailed down hardwood. The procedure is very similar to the nail-down process. Follow the adhesive manufacturer's instructions for installation and drying times.[11]
Choose your replacement hardwood flooring. You don't have to replace your old floor with the exact same wood or flooring type that you just removed! You can glue your new hardwood flooring into place if you like. You can also install nail-down, floating (or locking) hardwood flooring as a replacement. The choice is up to you and what your budget allows. Denver Hardwood Flooring
While the overall thickness of flooring is going to vary by category, it is still important to know what is contributing to that thickness. Engineered wood flooring can inflate thickness by adding more layers of plywood or particle board, but still only offer a thin veneer of real wood on top. The dense core allows for Rigid Core floors to be significantly thinner than traditional hardwood floors, with WPC floors being thicker than SPC products. That doesn’t mean all Rigid Core floors are made equally though. Just like how engineered hardwood flooring can inflate thickness, Rigid Core flooring can alter the overall thickness by increasing the size of the acoustical padding on the back. While this padding adds a lot of great features, it doesn’t need to be any thicker than 1mm in size. When it comes to Rigid Core flooring, the thickness of the core is most important. Denver Hardwood Floor Install
Another common hardwood floor malady is the development of gaps between planks. The most common causes of gaps between the boards is Mother Nature. Wood shrinks as it dries out, and it expands when it gets wet and humid. That regular expansion and contraction over time is the most common cause of gaps, and is the main reason why most gaps are seasonal in nature. Perhaps your best plan of action is to exercise patience.
Sanding provides a method for smoothing an installed floor, compensating for unevenness of the subfloor. Additionally, sanding is used to renew the appearance of older floors. Sanding using successively finer grades of sandpaper is required to ensure even stain penetration when stains are used, as well as to eliminate visible scratches from coarser sandpaper grades used initially. Prior to modern polyurethanes, oils and waxes were used in addition to stains to provide finishes.[7] Beeswax and linseed oil, for example, are both natural crosslinking polymers and harden over time.[8] Denver Hardwood Flooring
It is difficult to compare solid wood flooring to engineered wood flooring due to the wide range of quality in both product categories, particularly engineered. Solid wood has some limitations. Recommended maximum widths and lengths are typically 5" / 127mm wide and 7' / 2100mm long. Solid hardwood is also more prone to "gapping" (excessive space between planks), "crowning" (convex curving upwards when humidity increases) and "cupping" (a concave or "dished" appearance of the plank, with the height of the plank along its longer edges being higher than the centre) with increased plank size. Solid wood cannot be used with underfloor radiant heating.[5] However extra care is necessary with the planning and installation of the heating system and the wood flooring, such as limiting the temperature to 85 °F (29 °C), avoid sharp temperature fluctuations, utilizing an outdoor thermostat to anticipate heating demands, and monitoring the moisture content for the subfloor before installation. Denver Hardwood Floor
– If you want to change the width of the wood or the direction of the hardwood, the only way to do this is to get a new floor. The standard in home is oak 2 ¼” strip. Now, it’s more stylish and more common to get wider planks such as 3 ¼” or 4”or 5” planks. These look more modern and importantly make the room look larger. Another design trick to make the rooms look larger is to lay the planks on a diagonal – that way, it carries your eye along the longest length of the room. Or, you can even get fancy and add borders or do a herringbone pattern. Denver Hardwood Flooring
If your only goal is to brighten your floors and restore their original shine, consider refinishing them. But if you want to change some aspect of the floor itself, like the style of the wood, the direction of the planks or the consistency of the material, you will need to invest in a full replacement. Refinishing is ideal to restore existing wood, while replacement is ideal for establishing a completely new look or feel.
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