If your budget is firm on 15k, the best bang for the buck will be a full size uzi in my opinion.
A Mac may be the cheapest FA to get, no matter how you configure them, they will always be a little ghetto.
Stens are crude, like a Mac except they don't chew through the ammo as fast and they don't have the ability to be reconfigured or adapted like the Mac can.
15k can get you into a sterling, if you hunt a bit, but it's pretty much fixed in it's original configuration. A mini uzi looks sexy but it has a high rate of fire and unless you're into that, it's not ideal when ammo is expensive. And there's no rimfire conversion (that I know of) that's available for the mini. And unless you're lucky, I don't think a mini is going to fall into your $15k budget range either.
You can get a full size uzi for around $13k and it can be customized a bit with optic ready top covers, 3 lug barrels for easy suppressor attachment, folding or quick detach stocks, vertical foregrips, .22 and 45 caliber conversions and you can add rubber buffers in different thicknesses to tailor the ROF to your taste. As well, if you get a RR uzi that has been brought up to SMG spec, you can take advantage of the large amount of spare parts that are available cheaply for it. And magazines are cheap as well with surplus 32 round ones running around $10 a pop. Plus it's hard to beat the '80 action movie appeal of the UZI.
Just make sure you get a RR one with the blocking bar removed as part of the original conversion. You don't want to be stuck with a slotted bolt that's married to the receiver because the original converter was too lazy to remove it.