If you want get the longslide you can, but you don't need it to drop in a 6" barrel for hunting. I have a LWD 6" barrel (I got a good one, some have complained that LWD has produced some questionable quality barrels) that shoots the same zero as the stock G20 barrel. Like you, I wanted the extra velocity that the 6" barrel provides for hunting. You get a 100fps more velocity with the same 180gr load with the 6" barrel. The long slide is up to you, but you could be opening yourself up to a whole lot of reliability troubleshooting once you do that. As long as you have the patience to work out the bugs, you will be OK, and the long slide will provide you with the advantage of a better sight radius.
I also wanted adjustable sights (mainly for hunting) and tritium night sights for carry, so I chose the Meprolight Tru-Dot tritium adjustable as a compromise. I'm very happy with them. They have been plenty rugged for carry with no problems. Sights are largely a matter of personal preference, so YMMV. The front sight is large, but comes very close to filling the rear dovetail when held at shooting length on the stock G20 slide, so as long as your brain can pick the center of the front sight, they do align very well for accuracy.
The only other things I did to my G20 was a 22lb Wolff spring and steel guide rod set, a 3.5lb trigger connector, and a 25 cent trigger job. It's my favorite pistol. Enjoyable to shoot, very accurate, and 100% reliable for total carry confidence.
Congratulations on the new G20. I hope you have fun getting it the way you want.