If your intentions are for a 5.56 SBR, I recommend getting some trigger time on one first. I find them to be extremely obnoxious to shoot unless there's a suppressor attached. I almost did an HK53 build, but I went for the Steyr AUG A3 instead. Much less obnoxious, better ballistics, runs cleaner, and it was comparable in price. I find the AUG to be very natural to shoot, and in the case of the A3, the mag changes were the only thing I had to work at. The overall length is the same as my MP5 SBR.
Now, my MP5 SBR was absolutely worth the money, the stamp and the "paperwork." MP5 pistols aren't all that useful and an MP5 with a 16" barrel is a "shoulda grabbed my AR" proposition. In SBR format, it's a pretty formidable weapon; rapid fire hits at range are very easy, it doesn't blow out your eardrums, and when wearing its suppressor it sounds like a typewriter (yes, that stamp was worth it too).
You only live once, and we don't get to pick and choose when we have to return to the dust. If you enjoy shooting and you can get it done, make like Nike and JUST DO IT! If anything, I regret not getting into NFA items earlier.