Call Gun Connection, pretty sure he had one in there today. Ask for Dan, tell him you're a member here. 410-335-7252, he's also in our dealer's section.
If I were you I'd keep it simple. I mean I guess theres a difference between a tac build and a HD build, but ill tell you about my HD build.
Basic mossberg 500a with 18.5 inch barrel. Added a sling, a tac star side saddle for 6 extra shells, and ATI AR-type telescoping stock, and I added a surefire 6p LED (with an upgraded Cree LED module) to the end of the mag tube using a mount I got from J&G sales (works perfectly.) That's all I really need.
For HD, you really won't need a heat shield, arguable no ghost rings, and a lot of other crap that people put on. You probably wouldnt even need the side saddle but better safe than sorry.
That being said, when I do my TAC build in a couple weeks, it'll have a head shield, ghost ring sights, knoxx recoil reducing AR stock, sling, a side saddle for extra shells, some sort of light (havent decided yet), and POSSIBLY a red dot if i'm feeling really exotic :-P