There is really only two ways to approach purchasing your first "full-auto". 1) Buy what you can afford; 2) Buy what you really want.
What I mean by this is if you have plenty of discretionary money to spend, don't be cheap - buy what you really want. The process is a bit of a hassle and if you are only planning on buying one you might as well get exactly what you want. It doesn't matter why you want it - you just have to fully satisfy that itch.
If you are like me, and have some but not anything close to unlimited discretionary money, then you will probably limit yourself to F.A. basically around $7,000 and under.....or $4,000 and under. At the $7,000 and less level, you can buy:
lightning link - maybe
At the $4,000 level, you are looking at:
M10 and M11
Sten build (probably more than $4,000 now)
converted Demro Tac and other cheaper tommy-gun knock-offs.
Now, when you start talking 9,000 - $14,000, the real question is why wouldn't you spend a couple thousand more to get into an M16 platform. There is no question that it is the most versatile platform and there are numerous cheap parts available for mods, repairs, spares, etc.
Of course, there will always be some folks who just have to have this or that gun for their own personal reasons, and no other gun will satisfy the F.A. itch. I put Tommy-guns into this category. For less money you could probably get into an M16 and modify it to fire 45acp. But, that's not the point - it's simply not nor ever will be the same as a tommy gun!