Ok, 29er rims are the same as 700c rims with have a 622mm bead seat diameter (where the tire bead sits).

The rims come in various widths, with 29er rims somewhat wider and heavier duty than road racing rims.

The other measurement is "ERD", or "Effective Rim Diameter", where the nipples sit. This is not the same for every rim.

Each rim will have an ERD rating, and it could be different for different 29er rims.

So your Cube rim and the M-Wave rim don't necessarily have the same ERD.

The manufacturer should specify the ERD, although I like to measure my own.

There are several online spoke length calculators. They'll ask for the ERD, as well as the placement of flanges on the hub, as well as the offset, and diameter where the holes are. A disc brake hub may effectively be offset slightly, but you probably will have the same length of spokes on both sides (you should measure or compare).

If you don't have a caliper, get one at Harbor Freight.

Even if you only have the M-Wave ERD, ad not the Cube ERD, you can calculate your spoke lengths, and measure a few spokes that you actually have.

As far are reusing spokes. I've done it from time to time. If the spokes look good, and are the right length, then I'd be tempted to do it. Some people tape the new rim to the old rim and do direct transfers of spokes.

It still may not be bad to purchase a couple of spare spokes and nipples, just in case you need them.

The one consideration is that Sapim or DT spokes may be better spokes than the OEM spokes.

Anyway, building a wheel is more than simply installing spokes. You have to center and true the rim. Not to be taken lightly, but it is a good skill to know.

Post as you get closer to building your wheel.