Meltdown & Spectre bug & new CPU's
To answer your questions:


  Are new (2017+) CPU's affected by this flaw, too?     If so, when will
  CPU's start to come out that are not affected?


Yes.  It affects virtually every CPU going back to 1995


  It affects potentially all out-of-order execution Intel processors
  since 1995, except Itanium and pre-2013 Atoms.


As for when new CPUs will be released, that's impossible for anyone here to even speculate as it will require a hardware redesign.  Apple won't even be able to speculate as it's dependent on availability from the chip manufacturers.


  Will new (affected, unsold, still in shop) products have their CPU's
  replaced?


Nobody but Apple (actually all computer manufacturers for that matter) will be able to answer this.  Given that new, unaffected CPUs don't currently exist, it's reasonably expected that new machines purchased will still contain the flawed CPUs.


  My concern is that I will purchase my MB pro in March, just to find a
  short time later that Apple has had all products on the market
  recalled and their CPUs replaced.


It's a valid concern.  Nobody here can predict what actions Apple will take if any.  The best advice I can give is to take a wait and see approach given that March is two months away and there is bound to be further developments with respect to this issue.  If you need a new Mac for "work" (i.e. to generate revenue that pays the bills), bite the bullet and purchase it.  If this is something you can put off as it's not critical, I would take a wait and see approach.


  Apparently, some users, particularly users performing more "heavy duty" tasks, may experience a performance drop of up to 30%


According to an Intel Press release, they don't anticipate seeing any significant drop in performance for real-world deployments.  Chances are, you will not notice any drop in performance.

Intel has also released patches for these flaws and is working with OEM partners (like Apple) to push them out to customers.