For $6k, if you look hard, you should be able to find an IMI Uzi conversion. They work just as well as the Vector models, even though they're conversions. I had a Flemming converted IMI for almost 10 years and it ran like a Swiss watch. There are some differences between the conversions and the Group/Vector guns, but they are minor and can be worked around (mainly the barrel restrictor ring in the conversions).
However, if you can save up a little more cash, I'd wait and try to get a Lightning Link ($7000-7500 currently). This device works in your AR15 to convert it to full auto. It can also be used with a modified bolt in 9mm. Later this year when the LM-7 .22 beltfed upper comes out, you should be able to use that too.
If you're just looking for a pure investment, try to find a few SWD M11's for under $2300. Get them to a friendly SOT on form3's and wait 'till the price gets high enough you want to sell.
An ACC556 would also be a good move IF you can find one under $5600-5800. If M16 prices jump again this summer because of the LM-7, ACC's won't be too far behind.
In past years, appreciation on MG's has been 10-20% but the trend is definitely slowing as more and more potential buyers are priced out of the market. They'll still go up, but as they get more and more expensive, the rate of appreciation will slow even more.