I'm assuming $4k ish to spend (16" AUG would run that).
Go with the Vector UZI--the prices are about to go up, and you can still get one at $2900.
Alternate possibilities:
HK autosear ($4500)
FNC and S & H Sear ($4k total with 3 rd burst).
RDIAS for AR15.
I like my 20" AUG, but wouldn't sink the same money into it if I had it to do over again. HK 94 is not bad, but personally, I'd get an SP-89, as you can remove the folding stock on the MP5K PDW and the sear and have a regular pistol again, whereas if you chop a 94 to MP-5 and remove the sear, you need to register it as a SBR.
The Hot Tub--just run some hot bath water and fart a couple of times--we're talkin' GUNS here.
Right now, you could buy a NIB Vector for $2900, let it sit at your dealer (pay the $ for transfer) and sell it in a year for $3200 without firing it.
AFARR