OK, well first things first... What state do you live in and do they allow SBR's? If they do, are you willing to pay the $200 tax, send in fingerprints, wait 3-6 months, will your local CLEO even sign? If the answer to any of these is no, mentally add a 16" barrel to both of these and see how they look.
Issue #2 is the way in which would would need to build these guns. To get a semi uzi in that configuration, all you would need to do is buy a pre-ban and get the barrel chopped. You are going from a "rifle" to a "short barreled rifle", so no big deal.
The way you would build the MP5K-PDW is buy an SP89 pistol and add the folding stock to make it the SBR. Problem is that I believe the BATF considers going from a pistol to a short barrled rifle like that manufacturing a new firearm, and thus under the constraints of the 94 crime bill. This means no folding stock, flash hider, etc. even though your donor gun is a pre-ban. Now... I'm sure plenty of people have done this, but the legality is in question...
As far as cost, a pre-ban semi UZI is probably 1-1.5K depending if you get a model A or B, the model A's being more expensive. IMHO, if you're gonna spend the $1.5K for a model A and pay $200 tax stamp SBR it + gunsmith fees to cut/recrown or replace the barrel, you may as well drop the $3K on a NIB vector UZI (also +$200 tax stamp, of course) - they are probably the best value in the class 3 world right now (where else can you can a NIB machine gun with a factory warranty these days)?
An SP89 will go for 3-3.5K, plus the folding stock (not sure prolly like $100-$200) + K grip +$200 tax stamp. Good thing is that since the barrel is already of the proper length, you don't have to worry about replacing that. Still, you are likely in it for $4K before you even start thinking about mags.
Good thing about the HK is that you could always buy the auto sear later on if you do want to go full auto. Last time I looked they were going for about $4k (for a $5 piece of metal - it really is disgusting). If you get one registered in multiple calibers, you can then put it in any of the HK 9X series and basically have a "3 in 1" machine gun (G3, HK33, and any of the various MP5 clones). I believe there are transferable full auto bolts for the UZI as well, but I don't know what they go for or how reliable they are.
You could also opt for the post ban clones - vector sells postban UZI's with the folding stock welded open for like $600-700, I think. And Special Weapons makes a PDW clone which I think are wholesaling $1K (think FAC has them) with their new stamped receiver and a bit more for their original cast receiver version.
Either way, not the most effective use of your $$$. A semi subgun is more of a curiosity than anything else, IMHO. Unless you already have a pretty well rounded collection, I would recommend getting some other guns first.
Rocko