I have never used powder kits but have stayed with liquid concentrates. The one I use at present is made in Germany and comes I think in concentrates to make 10 litres of working solution (I always mix enough for the film(s) to be done then throw the used solutions away. It is marketed under the Rollie brand and comes from Germany. I bought mine from AG Photographic in UK. There are 3 bottles of individual concentrate plus one of 'starter' . Of the 3 only one will 'oxydize' so I decant this out into 50CC brown glass containers so it remains stable.
Powder developers I have always found, with B&W developer you have to mix the whole lot in one go and this is not best practice with C41 chemicals so I would stick with liquid concentrates.