I'm not sure where you heard that about the barrel lengths, but the stock barrel lengths for both guns are the same.
I suppose you could find some way to attach a vertical grip to an SD5, but it would be a hack and slash job that would pretty much look fugly, seeing as how a vert grip just doesn't look right on an SD5 (my personal opinion.) If you have some ingenuity, I'm sure you could find some way to do it and be different...
You have an option to put on the ICS rail system (meant for SD guns) onto the SD5, but it's an ugly-ass contraption, if you ask me:
You could upgrade the internals to give you better range outdoors, but short-barreled guns (such as the MP5 series) will experience diminishing returns as the power goes up. The amount of money you'd have to spend compared to another user with an upgraded armalite, AK or G36 to reach the same performance level would be greater.
If you're dead-set on using an MP5, at least get a longer tightbore inner barrel in there if you're going to be doing any power upgrades. It'll make the system more efficient. This is only really feasible with the RAS as you can attach a longer silencer to cover up the extra inner barrel length - which would be longer than you could get in an SD5.