You are using what most any trans shop would be using so no problem there. Seal kits are usually packaged with parts from various sources and sometimes I've seen OE branded parts inside. ( Auto makers "make" very little of the parts in a car beyond sheet metal / assembling engines / trans. )

Do a search of the forum, there are step by step instructions somewhere. The .4 is a smaller version of the .3 and the .5 has an overdrive unit. Are all similar enough that instructions will be helpful for all.

Rebuild the trans in sections, don't just take everything apart at once. ( RE: you can take all the major assemblies out of the case but don't break down each sub assembly until you have finished reconditioning each one. )

Lay the parts out on the bench in the order that they come apart. Pick a convention so whatever parts that are facing up are to be installed in a certain direction in the trans. In older trans, there are parts ( spacers , washers ) that look like they could be installed in either direction but have subtle differences and should not be flipped over.

Buy the ATSG manual for this trans. Don't bootleg it, this company puts lots of effort into making these very good inexpensive $ 30 books and sometimes even includes real world information. https://www.atsg.us/atsg/mercedes-722-3-722-4-dl.html

The source of the failure must be found and fixed. This is easy when there is a "hard part " ( metal parts that are generally reused in a rebuild ) but can be more difficult when "soft parts" fail ( frictions / seals ) . A set of burned clutches can be replaced to get the unit going again however, if the valve body has problems, the will fail again.

Speaking of the valve body, this is an operation all unto it's self so leave that for a separate job.

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