I recommend not going with the MP5SD configuration. You will be stuck with having to shoot only with the suppressor in most cases (I have a barrel extension for mine that allows it to be shot without the suppressor but it's dorky). It would be a three tax stamp gun. One for the suppressor, one for the gun since it will be an SBR (unless your suppressor is non-removeable and makes the barrel 16"), and one for the F/A trigger pack. It doesn't have to be an SBR but then you could never remove the F/A sear around (to other HK firearms) assuming your suppressor is removeable.
You are better off with an MP5, suppressor, and subsonic ammo. Most MP5's I know are HK94's SBR'ed, converted to full auto with a registered sear pack, and refinished and stamped properly like a real MP5. I have a dealer sample MP5SD and it doesn't allow me to use lots of the cool accessories for the front end like the RAS or special tube end laser.
Another friend I have uses a SW (like Bobcat) HK-94 clone, has it SBR'ed and chopped, and an E&L plastic SD extension/cover so it looks like an SD. Whatever works for him but silly to me.
If you ever want an MP5 and balk at the price of a registered full auto sear, it may be double the price in a matter of years and you'll kick yourself for not having bought one now.